But now the question isn't specific technologies it's in entire domains.

The counterpoint is that if I am proficient in Ruby, and senior level in Node, and work well in Java, I can figure out and be fluent in Python or PHP within a week or so.

Sure if you had someone that was working as an aeronautics engineer designing planes for years they probably would require more ramp up before being able to work on ICBMs, but they would do just fine moving from Lockheed to Boeing.