Majority of the ones I know are making billions. Why change something that’s already profitable?

Why make billions when you can make millions (not dealing with bad async primitives and type safety that doesn't need some third party project like sorbet)

Many start-ups (especially ones that aren't backed by millions of VC money) choose Rails because of the super high developer productivity and the broad availability of well tested packages.

You could just fade out rails as you could with any other monolith, but many companies aren't doing it because it offers legitimate advantages.

Nope.