Nothing is solely up to the foundation. Plenty of people have made it clear they would leave/fork if adverts were added to Wikipedia. If the foundation wants to continue to have projects to host, they need to listen to what their users want. They understand this, I think it very odd that so many users seem not to.
By users you mean more than just the handful of active users? If the active users who are freedom fanatics forked, their place would be filled by the many contributors who don't contribute now due to the amount of bureaucracy and philosophy that gets thrown around. Users won't abandon Wikipedia because of ads, they never abandoned anyone else because of it, why start with the single most handy resource on the web.
If there were two Wikipedias, one with ads, one without, which do you think readers would go to? The only thing the original Wikipedia would have going for it is brand recognition. It would take time for people to learn about the ad-free version, but people would learn (possibly quite quickly, since I can see the media jumping on the story).