Hosting wikipedia accounts for roughly 2% of their total expenses. I've been donating annually, but it would be nice to have the option for the entirety of my donations go only to running wikipedia and none of it going to wikimedia's discretionary spending.
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.)
