The convention could always select a different democrat. Or it could have if the democrats hadn't purposely put it after the ballot deadline in many states so Biden would already be on the ballot, before the convention that supposedly nominates him.
It's just not the either/or situation you're looking for. It's entirely possible to be so disgusted by the state of American politics that you just don't vote. Or you vote third party. Your memes are having a negative impact at this point. Nobody who was on the fence is going to say, "Oh gosh I don't want to be commie, I better vote Joe!"