This looks fairly lightweight and clean, but you immediately replace a large portion of the Rails ecosystem with React and will constantly need to account for that when deciding how to build your application. By sticking closer to "the Rails way" you get the support of it's massive community.

If Intertia.js development halts, then you're stuck with either a) adopting something else, or b) maintaining the tool for your own use cases. Using something like this would, imo, be closer to building a Rails app in API mode with a separated frontend than adding a new library on top of Rails.