This doesn't change my mind. They can change the license back for later versions, which would then require a fork.

It is also very telling that they didn't go with the Apache 2.0 license, which would actually be good for WordPress's use of React. Going with the Apache license would have been a much stronger move.

Apache License 2.0 explicitly lays down the grant of patent rights while using, modifying or distributing Apache licensed software; it also lists the circumstances when such grant gets withdrawn.

That part of the license could end up being important. The fact that no patent grant exists in the MIT license leaves a lot of room for Facebook to act here.

With the new license it still makes sense to move to something else, like Vue.