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.