Depends on the product. The Rails architecture is fine for CRUD apps, and Heroku will happily take more and more money for more and more database capacity. As long as your ARPU is solid, you're fine scaling like that.
But there are more things in the world than CRUD apps. I'm building tools to track hate on social media, and there's a lot of social media out there. Rails, or any RDBMS-centered architecture, won't cut it. And if you're not using Rails, then I think the case for using Ruby isn't very strong these days; Python has a bigger community and is much stronger in important niches, like ML. So we picked Python as our default language not because the Python runtime is vastly more efficient than the Ruby one, but because the ecosystem is a much better match for our needs.
I wish it were the other way, as honestly I like Ruby better as a language. But my personal tastes are ultimately a pretty small factor when I'm picking the tooling for a project.
For implementing ye olde business logic, which is far more common in software development than cool tech such as ML that is discussed a disproportionally high amount compared to its real-world usage, Ruby on Rails wins. Sure, there are lots of frameworks that are good for shipping business features quickly, but Ruby on Rails is still among the leaders of that group due to concise code, magical defaults that give you a boost, an excellent ORM, view helpers for sooooo many common scenarios and edge cases that the JS world is still playing catch-up to, etc. And a lot of the new defaults in Rails 7 are aimed at reducing complexity and rescuing developers from the insanity of front-end development.
Great for CRUD, yes, but plenty more than that. Soooooo many apps don’t need to wrestle with a huge amount of data or even a data warehouse or analytics or a data lake or any data science or anything requiring even a blazing fast query time on a super complex gnarly data model, and most of those would ship faster by using Rails, and be easier to maintain as well.
There's a lot of "ye olde business logic" in what I'm building, for people who often change their mind about what they want.
A Rails API (with Postgres and Elasticsearch) makes this fairly easy to handle, especially with the limited staff we have.
A Vue.js client provides the fancy UI the users want.
You and the person you're responding to are agreeing that you should use the right tool for the job, and there are plenty of jobs that Rails isn't right for.
If you will grow big enough surely you will need some kind of web presence - then you can introduce Rails :)
Or don't. Honestly having the team on one language makes a lot of sense.
Not really. ML in Python basically means lots of C++ libraries and you can just split processes as much as you want as it's easy to parallelize. You could do the same in R, Ruby, any scripting language really (but Python already has all the glue stuff ready to go and is easy).
But there are more things in the world than CRUD apps. I'm building tools to track hate on social media, and there's a lot of social media out there. Rails, or any RDBMS-centered architecture, won't cut it. And if you're not using Rails, then I think the case for using Ruby isn't very strong these days; Python has a bigger community and is much stronger in important niches, like ML. So we picked Python as our default language not because the Python runtime is vastly more efficient than the Ruby one, but because the ecosystem is a much better match for our needs.
I wish it were the other way, as honestly I like Ruby better as a language. But my personal tastes are ultimately a pretty small factor when I'm picking the tooling for a project.