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).