It has indeed.
It happened in real life to a guy called Oda Nobunaga. You may have heard of him.
http://www.samurai-archives.com/nobunaga.html - read 1570-73.
After the defeat of the Rokkaku Nobunaga was at war with the Asai, Asakura, Ikko Ikki, Miyoshi, Takeda, Uesugi. After Honno-ji Hideyoshi fought the Akechi, Shibata, Tokugawa, Chosokabe, Shimazu and Hojo. Both lists are about as many if not more enemies as you'd expect to fight under RD in the game. Just because everyone declares war on you it doesn't mean all of them will be sending troops against you.
False. RD happens gradually. There is an initial penalty which increases turn by turn.
They are putting down their quarrels to restore the status quo, which you are endangering. Nothing special or abnormal about that really.
I don't use darthmod, but most times steadfast allies stay on your side for a while, especially if you butter them up, pay tribute, arrange marriages, the like. If they agree to join your battles when everyone declares war on you they get diplomatic bonuses in your favour.
It also has historical basis in the case of Japan. Convenient - maybe. Cheap - No.
False again. It represents everyone hating Nobuaga and Hideyoshi by association in the period from Honno-ji to the latter's unification. Sekigahara didn't happen until 1600, or right at the end of the campaign date.
I disagree, but then it's just my opinion against yours. Oh wait I have history on my side.
I don't disagree the game could have represented the realm divide dynamic better. I've posted several times in the past you should be able to choose steadfast allies as coalition partners amongst other things. That said calling it ruining the game is a bit rich.
RD is in RotS too.
History 1. You 0.
Radious has several mods that dilute or remove RD.
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My bottom line is this - we all know it's there. It's far from insurmountable as a gameplay mechanism, and with some planning ahead (save up money, build up troops, weaken clans through subterfuge, learn how to play siege defence, etc) before your clan fame hits legendary you can prepare quite adequately for all of Japan to hate you. I've completed legendary Takeda/Uesugi campaigns with no vassals or allies past RD, and I don't even consider myself a great campaign player. There are guys on this site who can do things that boggle my mind completely, so I can't imagine they'd find RD difficult to deal with either.