FWIW they have instructions for downloading the whole thing, which mention that it is also downloadable from BitTorrent. So I think it is functionally impossible to delete Wikipedia.
I’d be more worried about propaganda being inserted.
I'm not worried about the data, and even losing the servers would be a hiccup. But whatever site is at www.wikipedia.org is, in the minds of the general public, Wikipedia.
I imagine most traffic to Wikipedia is through search, so I imagine such a fate is in the hands of search engines. If a community-driven alternative appeared, we'd have to rely on Google indexing this alternative and ranking it higher than the usurped domain.