My issue is that Monstropolis is a modern city, while the area around it would be the 1920s/30s. So its not as clean a transition as some are thinking in my opinion, especially not based on the concept art for the WDW version. Could it work, of course, but so can Avatar with a portal/tunnel.
Also if Monsters Inc wasn't there would anyone really be pushing to have Monstropolis in the Backlot, I have my doubts many would.
As for the Monorail, that is already being moved for the Eastern Gateway anyways. So that isn't an issue no matter what is replacing the Backlot.
DCA, as has been discussed many times now, has dropped a majority of that theming especially recently, so I think its sort of out the window of things "not fitting" anymore, at least in my opinion.