On the flip side, I recently worked on a "hybrid" project that has been around for a couple years. It was a nightmare knowing when SSR stopped and SPA began. There was no consistency, multiple ways to do the same thing, and poorly maintained. It was schizophrenic.

I think the real issue you are dealing with is what others mentioned: rebuilding something from scratch burns a ton of people-hours and money. Asking a bunch of rails developers to lean into FE development is a recipe for animosity and to me it shows in this post.