The WMF does quite a few things besides paying for the hosting costs of Wikipedia. As one example, their latest project is Wikidata, a general-purpose, language-independent knowledge base which already gets more edits over time than the English Wikipedia does. Given that developing such a generally-useful knowledge base has been a "holy grail" of AI for literally decades and that Wikidata is freely available at zero cost right now, those donations might already be more than paying for themselves.