The thing is, the price I want to pay is "what it takes to get the game made." If that means $5 a month for two years and I end up paying $120 for a game everyone's going to get for free? Honest to God, I am 100% OK with that, because I have supported a small business. I'm all about supporting developers.
It doesn't start being a problem until the developer starts slowing down, working on other projects and then quits. Whether it's because they got tired of working on it, or they had health problems, or they had "health problems" (put in quotation marks because I know this is often code for "I just really didn't want to do it anymore and figure no one will question me if I say this"); and then whatever I've put into it is just gone and I have shit all to show for it.
The "pay per update" model is certainly an option. As I recall, that's how Patreon actually started. You set an amount you paid per update, up to a limit. The problem there being that it puts pressure on the developers to rush things out the door before they're ready, and you get a stream of crap updates. I dunno, maybe that would be the best way of doing it. Pay per major update; even if it meant more small updates and bug fixes; it'd at least let your sponsors know how you're progressing.