tbh, you can't really argue either way for the cohost, since some do a lot, but some do very little, while some games are just fine with one host.
My two games I did an almost equal amount of work in RotS in role pms and design with Yeti, and ended up doing almost everything for TTV (with some notable exceptions), so I can't really say a cohost definitely helps.
It could also be argued in cases like MUDS where if there had never been a cohost, the mistakes may not have been made, although it's too early to say if it has made the game significantly worse. However, it would be best to emphasize that all hosts are human, and we all do make mistakes, so it's not necessarily true that the mistakes may not have been made if there was no cohost.
Either way, to try and get this back on track, could someone point out a game where there was a legitimate mistake made by someone who had not really ever played mafia before on this site that led to disaster, besides some of the earliest games?
Before anyone mentions Luigi, there weren't that many options other than US ot sub in, and he would've likely pulled the scheme no matter what he was. It could also be blamed on the villagers for trusting him so much when he had such a horrible role for leading, even with the Dickens attached.