https://en.wikipedia.org/wiki/Wikipedia:Database_download#Wh...
Mirror it and make it available read-only until that day comes as a separate "disaster recovery" organization and raise the minimal funding required for that function (1 employee + 1 dedicated server) would be sufficient to the task. More if you wanted to make it usable but at that point, you aren't really a dumb mirror.
> pages-articles.xml.bz2 – Current revisions only, no talk or user pages; this is probably what you want, and is approximately 13 GB compressed (expands to over 58 GB when uncompressed).
You just need to mirror this regularly and maintain a reasonable depth of the backup. (Say, once a month for the past 12 months)
Then, whenever this terrible event destroys Wikipedia you are on the clear due to you operating under the past licenses for the data and people will need a new place to go if Wikipedia is genuinely destroyed.