The reason why Node.js cuts down on development times is due to the fact that you can have the same engineers working on both the frontend and backend - Basically it puts you in a situation where you can have a single engineer own a feature's development end-to-end.
When you have a company which is split up between frontend and backend teams, this requires your engineers to coordinate their work with one another and this reduces overall productivity (you usually need a lot of back-and-forth to bring the feature to completion).
Yeah, but that narrows down your pool of developers a lot. There are a lot more JavaScript developers than there are JavaScript developers who also happen know RoR.
I mean, sure, someone could learn RoR on the job, but in practice, will they actually do it? In all companies I worked for where the frontend and backend languages were different, developers tended to stick to one side of the fence. Developers just don't like context switching.
In my experience any developer worth hiring can do complex frontend work (even by 2015/2016 standards) and backend work. I've never seen a "frontend only" or "backend only" developer.
On the other hand, if a developer cannot figure out Ruby on Rails (or Sinatra, or Django, or Flask, or any of the PHP frameworks), and does not even know SQL, do they really have any business doing server-side web development?
Knowing JavaScript and one server-side web app framework/language is a pretty minimal requirement for a full-stack web developer. It boggles my mind to think that there are people working as professional developers that literally only know JavaScript. Those people should not be doing server-side development.
It take longer to adopt and learn RoR than ES6. Both have their owns strength but Javascript and Python skills can be apply to any organizations, RoR is quite limited.
There are mature, production-ready server-side web app frameworks with relational database adapters in literally every mainstream programming language (and many of the obscure ones too) except for JavaScript. I didn't say you have to do Ruby on Rails. If you prefer Python, then you can just use Django, it is more or less equivalent to Rails.
When you have a company which is split up between frontend and backend teams, this requires your engineers to coordinate their work with one another and this reduces overall productivity (you usually need a lot of back-and-forth to bring the feature to completion).