A Fork of Wikipedia wouldn't face the insurmountable network effects. You'd have all the content ready and waiting.
Losing even a small proportion of contributors could seriously affect the future development of Wikipedia.
The sale or takeover would start with promises that nothing would change, and then would change slowly enough that each editor would have a different breaking point and thus be too fragmented to move over at a specific point.
Wikipedia has been slowly losing editors for ~12 years[0].
[0]: https://commons.wikimedia.org/wiki/File:Active_editors_on_En...
A fork of Wikipedia with no community is just a content mirror and, because of network effects, you'll have an incredibly hard time attracting anyone to help out with it when they can just go to Wikipedia.