I just speed read though it, So basically Everything is changed? I am not even sure if that is like a re-architecture, it is more like a a rewrite, literally every apart of the stack, all the way to Javascript Engine. All aiming for 2020 release.
At least the whole stack have many large companies'vested interest in it now so it is unlikely RN will fade out anytime soon. Will be interesting to see how all these played out in 2021/2022.
They will probably slowly introduce the changes one feature at a time in a backward-compatible way in a similar manner to how core react architectural changes are introduced (eg fiber, suspense, hooks etc).
At least the whole stack have many large companies'vested interest in it now so it is unlikely RN will fade out anytime soon. Will be interesting to see how all these played out in 2021/2022.