However for pretty much any dev I would hire for a job they can get to grips with a technology that's older pretty quickly. Where it does get dicey is when good dev just refuses to work with it. For those devs, I think, when they hold that opinion it typically means one of those other reasons is behind their refusal.
I mean, one of those reasons is "When I leave this job will I be able to get another job" is a huge deeply life affecting one.
If you want me to work on COBOL from 1988 then you've limited my work prospects to one of a very few employers in the country at a very specific pay range. If I instead tell you to eat a fat one and go with $language_de_jour the number of employers and potential salary range is much, much larger.
I know devs like to say they would hire anyone, but they're not the ones hiring. At best you get to interview people already prefiltered by HR which... looks for keywords in CV.
I've never found it too difficult to get hired even when the requirements don't list something I've done already.