I agree with that; I think people undervalue generalists, get stuck with a specialist which is nice while you are in that particular field, but the world changes. Specialists are specialists because they spent a lot of time on one thing; depending on your age that means you didn't spend a lot of time on anything else and that might turn out to be VERY difficult when you get older. I don't know many PHP 'specialists' above 30 who can actually 'change' their calling. They have an extremely hard time picking up anything else as they have been coding PHP (and a tiny bit of JS) only for the last 15+ years. It takes serious effort to switch at that point if you're not used to that flexibility in your brain.

Keeping up with the market is a big thing though; as someone who was in the dotcom boom and who did backends at the time, I only noticed that everything changed years 'too late'. It wasn't really too late, but my eye wasn't on the ball for sure as I had a cushy position anyway I didn't need to, but when I sold my stock and went looking for the next interesting thing to do I noticed that the world changed significantly.