I disagree that learning any new tech is useless in general because you found two cases where it was. Any old tech was new at a point. What matter is learning new ways of thinking (paradigms?).

Then you see that syntaxes and framework API changes but it’s not a big deal because you already know what you want to do, look it up in the docs, and can pick up new languages in a few days.

For example learning high level concepts like filter-map-reduce with the appropriate data structures is relatively language agnostic, so I have the same reasoning between languages and all I see is syntax choices. It greatly improves learning speed and then I am free to choose the best tool for practical considerations for each project.