Surely a lower volume fork of wikipedia could somehow incorporate upstream changes in a more or less automated way? It seems like a site serious about doing this would work on some kind of moderated conflict resolution system for when it does happen, but the vast majority of pages would presumably not need it.
It's in the works. Forking something the size of wikipedia is a serious undertaking, not to mention the rickety pile of code it's built on. It's still early days.
The Chicago Cubs haven't won a World Series since 1908: https://infogalactic.com/info/Chicago_Cubs
Without enough users, a snapshot of Wikipedia will quickly decay into unreliability. Most pages don't become inaccurate that fast, but you can't know if the page you're reading is one of them or not.