SPAs are one of those things that few apps truly need. And honestly, these days, if I thought I truly needed an SPA I'd probably just write it in a compiled language that spits out WASM.
I'm honestly surprised it didn't take off massively for internal enterprise CRUD stuff. It's by far the most productive stack I've ever used, the second closest being .NET w/ EntityFramework.
It was the darling of the early SaaS explosion of the early 2000s, and then just kind of died off in hype.
I like that it's opinionated, stops be from going down rabbit holes, and batteries included. There's nothing in the JS/TS framework of the day world that leaves me wondering "hmm, maybe I should stop using rails for this." It all seems worse, somehow, despite being more "modern."
It's difficult for enterprise to bet against C# or Java and I kind of get it. Following the crowd is a form of hedging your own career, if things go south you won't get fired for it, but if you bet on an obscure language and framework with no support you will.
God, I had to deal with a junior we hired that wanted and attempt to SPA all the forms on a intranet application form.
I struggled to explain I would now have to spend 4 hours updating dependencies, maintaining a node js pipeline and dealing with typescript all because you couldn't handle adding a new fucking single form input to a form with only 3 existing inputs, which would have taken only 3 lines of code between the HTML and C#. The cake was this is an intranet app where the entire form only gets used a few times a year at best.
But Rails is hard to beat for CRUD apps.