I mean, they're currently getting over 50k per month, they'd have to fuck up significantly to drop that to a level which wouldn't be considered good.
Sure, but it's likely that this kind of support will start to drop quite a bit over time, especially once the main story, the school girls, etc. are done and once we are talking about some final details. Given that DC has been talking about "his next game" for quite some time, he'll probably start something new at some point to keep people excited. That said, the game does indeed have a very broad support base and many people only pay small sums, which many of them will probably continue to do, so we'll see.
Edit: It's not that I'm saying he won't finish the game. He'll certainly do what he promised on his development roadmap, all I'm saying is that one shouldn't have too high expectations beyond that.