Range of employment options. Possibly salary, though that's more variable. There are some jobs keeping the lights on with legacy tech long after it is done being the hot thing, but typically with any particular stack it's a shrinking numbwrt of jobs often with shrinking average real pay unless it hits a phase where the decline in people able to do it exceeds the decline in work.
If you are riding out the last few years of your tech-focussed career (whether that's before retirement or before moving out of hands-on tech into, e.g., management) that's maybe not so bad, but if you planning being in tech for a longer period it's potentially extremely career-limiting not to adapt to current market focus.
I'm not sure this is true. Most of the shops I've been in don't care about whether you know this or that language or library. You're expected to learn that as you need to. Most of what I've seen cut people from interview loops is missing fundamentals.
Then you'd be wise to stick stuff like Java or .NET, because there are probably millions of jobs requiring them.