But if the Foundation has to resort to ads because there just isn't enough money coming in from donations and other sources to keep running otherwise, how is the ad-free fork going to manage any better?
I don't support ads in general, but if the choice is between Wikipedia with ads and no Wikipedia at all I say stash a backup of the database somewhere safe and then go for it.
If it ever gets to that point, then you're absolutely right. Wikipedia with ads is certainly far better than no Wikipedia at all. We are nowhere near that point at the moment, though.