I do think it's hard to draw the line there, just because "running wikipedia" is kinda fuzzy and could mean different things to different people.
There's pure hosting costs (servers and bandwidth), then there's staff to maintain those servers, staff to handle legal issues around a large community-contributed project, staff to do moderation and other community-relations work, staff to improve the mediawiki software that wikipedia runs on, staff to manage administrative stuff for those other staff, etc.
There's room to debate which of these are necessary to "run wikipedia", and how much of each of them is needed, but it's a lot more complicated than just keeping the servers turned on. (And if you do think that the Foundation could be just-paying-for-hosting, you're implicitly hoping that some people are going to be donating their skilled-work for maintenance.)
There's pure hosting costs (servers and bandwidth), then there's staff to maintain those servers, staff to handle legal issues around a large community-contributed project, staff to do moderation and other community-relations work, staff to improve the mediawiki software that wikipedia runs on, staff to manage administrative stuff for those other staff, etc.
There's room to debate which of these are necessary to "run wikipedia", and how much of each of them is needed, but it's a lot more complicated than just keeping the servers turned on. (And if you do think that the Foundation could be just-paying-for-hosting, you're implicitly hoping that some people are going to be donating their skilled-work for maintenance.)