I agree with your point overall but disagree with one point: >Your co-worker's path is dangerous. Replace "Microsoft" with "COBOL" or "Fortran" or "Mainframe" and you'll see that this is a problem.

The coworker is described as being "in his fifties." It's probably not unreasonable at that point to have significant depth in a single ecosystem. COBOL was exactly the case I was thinking of: very few (if any) new COBOL programmers are being minted any more and there is still a lot of legacy code. It will probably be a good, if dull, career for the next 20 years for that coworker who will at the end be able to write his own ticket to fit around his almost-retired status.

For someone with 40-50 years of working life ahead of him/her, this sort of specialization is indeed premature and dangerous.

For me that kind of specialization has never been attractive, an attitude that has served me well. But that's not true for everyone, at every stage of life.