Look no further to be honest; look at older generation programming languages like COBOL and how sought-after good developers for that language are.

But I'm also afraid / certain that LLMs are able to figure out legacy code (as long as enough fits in their context window), so it's tenuous at best.

Also, funny you mentioned HTML / CSS because for a while (...in the 90's / 2000's) it looked like nobody needed to actually learn those because of tools like Dreamweaver / Frontpage.