I wonder, if it was $120 for the hardcover, would people still be complaining about the Patreon? Then it would be the same price. I, for one, backed it when it launched because if there weren't enough patrons, it wasn't going to launch at all. The monthly fees, presumably, pay the authors, editor(s), artists, and graphic designers for each issue (after Patreon takes their cut, anyway); the hardcover doesn't have to do any of that because they have already been paid. It's just a nice collected volume of everything you have helped make happen. If enough people stop pledging, then there won't be any annuals, either, because there won't be any GPG in the first place. So you're welcome for the privilege, I guess, but do keep in mind that the money finances completely separate things.
The problem here, if there is one, is probably due to the nature of Kickstarter in general. GPG is funded through Patreon, not Kickstarter, so how would they know that you have been backing it and for how long? Can they issue promo codes that you can use? If so, then sure, maybe it would make sense to give people a cheaper rate, but I don't know if that's even how it works and it's a lot more effort to manage. It would probably be easier to do all that if they just released the book without Kickstarter, but I imagine the whole point of doing Kickstarter was because they don't have that kind of money sitting around.