Your post is a bit misleading. Rails can scale but to get there is different than with other stacks. Some would say, it's much more challenging or complicated, some would call it just a PITA and if you really, really want to scale, like e.g. a porn site, Rails is the worst choice you could ever take. Or you would just completely rewrite it but what is then the motivation to take a framework? Maybe frameworks are wrong...?

However, I think dev productivity is more important since scaling is a requirement which is very often not needed and introducing it too early is just premature optimization.

Still, I dislike if people spread false information. Rails can scale to a certain (limited) degree but it never was easy, just the deployment then without Docker was a big mess (I didn't know one Rails guys who could deploy anything without Heroku).