React has been carefully designed to encompass better UI patterns regardless of the underlying platform. There are some very thoughtful and capable people working on it. Worth reading Jordan's thoughts here https://news.ycombinator.com/item?id=8962171

Expansion != abandonment. In fact, were I a "competitor," I would be concerned about the philosophical buy-in that said expansion could induce.

It's not? I must have missed that announcement.

I think you misunderstood. they are adding a non web library, not leaving the web.