Well, my long-term experience is different. The time you've spent on old languages is never really lost.

Every language that you learn and abandon, makes you understand the next one much quicker. Furthermore, you learn the appreciate the new constructs and idioms that new language brings. The new language usually also supports all the old constructs that you got used to in the past. So, you can always fall back on those too.

Concerning computer science, and especially compiler construction, indeed as you wrote, it applies equally everywhere.

After six years of relatively easy money of mostly delegating PHP work to others, I moved on to nodejs this year. I am not that keen on delegating node work at this time, because I want to play a bit more with it by myself.