Total War campaigns have always really been there to give context to the battles.
However what CA likely found is that unless a battle is close in auto-resolve people generally don't play them.
Thus in Rome 2 they decided to require generals to lead forces and have static garrisons. The plan was likely to have more battles that mean something.
While Rome 2's campaign was streamlined I don't think it was "dumbed down." You have more movement mechanics than Med 2, with less building slots you have to make choices on what to build*, and the politics system is far more
advanced. In Med 2 you couldn't even set your heir.
Battle AI isn't exceptional but pathfinding is better than before and while the battles aren't "meaty" there's certainly more thought put into abilities and formations**.
*To be fair it's a no-brainer to convert cities deep in your empire to pure economy but at least early on you have to decide if your port is gonna be military, food, or trade oriented; to name one example.
**Okay Warhammer ditched formations but in exchange we got lords with powerful abilities to either support or do damage, abilities that greatly buff the armies on campaign.