What happens after the relicensing isn't measured as the community has already been disrupted. I'm also less certain on how to fairly measure and predict fork likelihood.
One challenge for forks is that relicensing doesn't always jump from fully-open to fully-closed. There's a lot of "fake open-source" and source-available licenses, like the one Redis now uses. These may "only impact you if you are AWS", so a fork "that AWS can use for free" feels less compelling. If React was to relicense, I expect they'd similarly take a small step.
One challenge for forks is that relicensing doesn't always jump from fully-open to fully-closed. There's a lot of "fake open-source" and source-available licenses, like the one Redis now uses. These may "only impact you if you are AWS", so a fork "that AWS can use for free" feels less compelling. If React was to relicense, I expect they'd similarly take a small step.