[tacostakohashi]

I don't think the analogy really works, for a few reasons.

The first is that you've already (accidentally) timed the market by "buying" PHP in the late 2000s when that was dominant. There's an illusion that everything that existed when you got into the industry is somehow dominant, fundamental, and long-lasting, and everything that comes after is a transitory fad. That might even be true for some things, but many things are transitory fads, and PHP was one of them - the problem is you didn't sell / rebalance as it was declining.

Secondly, getting into / out of a technology can be a gradual process, more akin to a dollar cost averaging strategy. If you start to learn Rust, your PHP skills won't vanish - some of them will even transfer, and others will fade, but still be there if you need them.