For your specific situation, rather than a general does it make sense, this is a reason why things should always be written out as a contract and these things should be considered beforehand. Assuming you don't have a contract that agrees all this, which it sounds like you don't, it seems to me you have a few options, none over great:
- say you want more pay and if not you'll leave
- start your own Patreon and say you simply won't do any extras beyond the earlier updates
- tell the writer to start contributing to the Patreon, like short stories or the like
- continue the argument
In many ways you're in the stronger position, since the your comic is already out there and if you refuse to do any more, the writer will have to find someone new. On the other hand abandoning your work can get you a bad reputation. You don't wanna be the "they forced more money out of me part way throw and held the comic to ransom" person, after all. It's a tricky situation and a reason why people always warn to think about these things before you start a collaboration. Too many people go into them with a bit too much optimism.