Then that's when you should definitely start looking to specific solutions to specific problems.
Are you having scaling/performance issues? Look into kubernetes.
Are you having problems with "too many cooks in the kitchen"? Start looking splitting things to separate services.
Are you having problems hiring "full stack" people than can work on both ruby and frontend? Then start thinking about an SPA.
That's the power of Rails.... you can get really, really far with it... when your business is a total success and you have steady income, then start looking into the specifics.
Starting from the get go with an SPA, kubernetes, microservices, monorepos, etc is, in my opinion, a recipe for failure.
(not that you have said that of course, just something I see too much around and I just take every opportunity to rant against it )
Your dichotomy of Rails vs a bunch of complicated stuff aren't the only options. We can have something like Rails minus a bunch of magic in a performant language. Maybe it's a little bit less productive at the beginning, but you're not hating life in 18 months when your team reaches 15 developers or you've got to scale.
Java/Spring's magic is just as bad with annotations and XML, but you do get better performance and types. It'd be my second choice.
Things that are by far superior in Rails in my opinion:
* It is a real full stack solution. From migrations to css to credentials storage to assets compilation to file uploads to rich text editing to we sockets, etc.
* Ruby might not be as popular as python, but it is a lot better designed (and a lot more consistent).
* Gems/bundler and everything about packaging works a lot better.
* The routing, controllers and validations make a lot more sense to me
The only thing I like more from django is the built in admin. Although there are also external gems for rails which provide same functionality for rails.
I previously had a webapp built in Django, and just couldn't grok it. I made it work but always felt like I was fighting it. I tried Flask, but you have to build absolutely everything yourself.
I have a tiny team (2 devs) and hate spending time building and maintaining cookie cutter functionality like sign in flows, teams, etc.