The premise that you get meaningful efficiencies from JavaScript on the back-end just because you have to use it on the front-end has been pretty thoroughly debunked at this point. Instead you mostly get a larger blast radius when the front-end ecosystem has its monthly identity crisis. OP's "stacks-du-jour" and programming language "flavour of the month" framing is exactly right. A shocking amount of web software architecture is just following fashion trends dressed up as technical decision-making.
Most of the churn in tech stack isn't driven by engineering requirements, it's driven by résumé optimization and Hacker News anxiety. Rails has quietly been powering serious businesses the whole time. Does anyone think NPM's 3.1 million packages enable more functionality than RubyGems' 190,000 packages?