This is not about a learning curve, I feel like you will never be productive with any of those javascript framework.

Yes, of course there is a learning curve. For react you will be a lot faster after a few month using it. And this curve is a lot longer than for rails(react and rails are just here for example here).

But it seems even after being a react god master you will still be x10 times less productive than an average rails developer for developing an everyday feature.