My assumption is that the co-founder would need the investors to vote to fire you? As in, he can't do so himself? So is his threat that he will leave if you don't leave?

If he leaves, can you run it yourself and do you want to do that? My guess is that you would have to bring in someone to do whatever the co-founder was doing.

Your equity ownership is worth something if you have 60k users. You have a subset of them paying something already it sounds like. This is a great thing, if you enjoy it and believe in it. If that is correct, then if your partner has a problem, and you think he is replaceable, then you don't seem to have to do anything related to his demands.

I think this is also a good lesson in needing to have a really good relationship with the investors as well. If it is correct that they are the swing vote on decisions, you want them to appreciate you, know you well and understand the value you add.