As much as I support the decision for game balanced to 300 turns and resulting 1 turn per year pace the statistics from Steam aren't very solid. I'm sure there are far more reasons for CA to develop and balance toward 300 turns but using statistics from Steam is a bit misleading for 2 major reasons I can think of easily.
1. most of the TW games on Steam are balanced to play less than 300 turns, usually quite a bit less. I rarely play Shogun 2 more than 80 turns and have 3/4 or more of tech tree done in that time so certainly within 120 (since usually control more research point producing buildings or have more generals the longer game lasts research goes even more quickly) the entire tech tree would be researched. Also within 80 turns usually 2-3 castles will be nearly fully upgraded (out of 20-40 controlled by that point so that does leave considerable buildings to upgrade if I wanted but Shogun 2 doesn't offer much benefit for that and actually with the food mechanic upgrading all castles would be maybe even counter productive). NTW is about same as Shogun 2 while I have played ETW campaigns longest of all recent TWs close to 200 turns I've never reached 300 because entire map was conquered before that. How many people play past conquering entire map (actually it might not be possible if all factions are eliminated but I only did that 1 time and can't remember).
2. How many people play short campaign vs long campaign? That is a Steam statistic I'd be curious to know. I almost always choose short myself because I prefer to set my own objectives anyway though I'll accomplish the short campaign along the way doing the long campaign might take me away from my personal objectives. That said- I hardly every have the campaign objective to conquer the entire map which is the only way I can envision taking 300 turns and since CA balances games for less than that amount which means move distances, income, techs, diplomacy modifiers, etc all make possible conquests in less time than that which means only very slow turtlers might reach 300 turns- even more is the way end campaign plays out where an Empire that is 20 regions might conquer 3x as rapidly as an Empire of 10 regions... so an Empire of 40 regions conquers 9x as fast as an Empire of 10 regions which means conquering entire map actually speeds up the closer you get. Most people have already fulfilled both short and long campaign objectives and their own personal objectives long before 300 turns because CA balanced previous games to less than 300 turns.
That said there are many other good reasons I can think of that outweight importance of Steam statistics to have it 300 turns.
1. 117 factions. Even if 30 of those factions are eliminated in the first 10 turns if CA has seriously made more emergent factions, rebellions, etc then each faction will take some time to process. Even if most factions are less than 2 seconds to process (not something I've ever experienced in TW game) that is nearly 2 minutes of wait per turn. 150 turns x 2 minutes = 5 hours of just waiting for turn to process. 300 turns would take around 10 hours of just waiting not including actual time to make campaign map moves and fight battles. Steam probably has some average turn length time statistics but I'd guess average is at least 20 minutes if not longer. So about 110 hours to reach 300 turns of which 9% is just waiting. Considering that the main argument for more than 300 turns is because people want to be doing more that means more content CA has to figure out how to add and balance to extend the value of that play time. Otherwise making 600 turns is just an empty goal if only 300 turns worth of play is doubled. Few games give more than 40 hours of content these days so if even with goal of balancing for 300 turns- that is balanced on easier difficulties which CA also said most players use. (that really surprised me- as a point of pride I rarely can stand to play less than the most difficult setting, I thought more players would be at least not going below normal/average setting). So many veteran players can probably complete campaign on average 150 turns or even less for easier factions. The veteran turtlers will be having to play so slowly to wait 150 extra turns but maybe they will have fun doing it, vast majority will not.
2. Action vs waiting... to balance game for say 600 turns to at least give 2 turns per year means travel distances are probably halved along with research times, etc. If CA doubles the research tree potential that is bigger budget and more balancing. If CA merely doubles the time each tech takes to research there begins to creep in alot of empty time. Empty time is when players start to think the game is a bit boring and maybe they should be playing something else. Some players can amuse themselves pretending to be god or Alexander or something but few can do that for 500 turns in a row at or 15-20 hours worth of waiting for turns to process. I actually believe CA is testing the waters a bit to make such long amount of turns with content balanced towards that. Rome 1 might have had 500+ turns but by turn 200 everything that could be done was done aside from turning map your faction color and maxing city populations. CA is saying that at 200 turns most people will still have stuff left to accomplish- research, politics, intrigues, rival faction alliance blocs or emergent factions and rebellions, possibly even new unit tiers not unlocked (as opposed to unit hero as in Shogun 2, in R2 it might be imperial armies not even used if Rome was Republic first 150 turns).
3. GAME DOES NOT END AT 300 turns. If you want to take 600 turns to accomplish your goals you are able to. There will just be many turns of empty time that CA doesn't want to force majority of players to sit through and dream of better game design. So in reality CA is offering to most of the turtlers what they are asking for. Only real sacrifice is in general's life expectancy but if CA has made generals develop 3 times faster generals will still become epic- it just means players will have to develop more epic generals per campaign. Which sounds more interesting and fun to me. What happens at turn 100 in a 1200 turn campaign (4 turns per year) when your general is maxed out in epicness? (aside form a very few random traits) You get to look forward to another 100 turns with that general not advancing much or doing anything interesting except slogging through battles. Oh- but CA could slow down generals development... sure- if you don't mind general gaining some small new trait/skill only every 12-16 turns. Hell, most people would probably just get in the habit of hitting end turn and skip right past some content without even realizing.
4. Mechanics... what does 4 seasons add to mechanics? Possibly winter attrition but how authentic is that? Only if army is out of supply or in poor location but then why does it only happen in winter? Army can be out of supply or in bad position any time of year. How much of the campaign map is going to be Russia if it was even as cold in Roman era? Campaign map visually changes, income could change, currents/distance navies travel could change. There are quite a few things CA could do... but require changing the underlying engine and mechanics more which goes back to budget and franchise. Every time CA adds something they risk screwing up a good thing, adding enough new things to keep people occupied for 1200 turns is asking for poorly tested buggy crap (the beta testers would be taking significantly longer to test things and more potential to miss something which either delays release of game and adds costs as well gives lower review scores and these forums filled with rants about bugs instead of number of turns). The hard place for CA is that if they don't change some things every release the franchise grows stale, especially for me the battles are at risk of that. Still CA has to take time and be careful what they add... the list of games which added too much and changed core mechanics alot and lost support is alot longer than games which stuck to a core philosophy though there are only few franchises that even got close to TW length of success. Civilization is about the longest running I can think of, Starcraft, AoE, Final Fantasy, Mario, FIFA, Madden, Ghost Recon, Tomb Raiders, etc there are a few but more often we get Duke Nukem or Star Trek, Wing Commander, well... just about anything with at least 3 titles that never reached a 4th.
