What’s your point? Technologies come and go. People built Ruby apps that did their job and now good luck finding maintainers for them. It’s just how our market works. We’re lucky if our stack lasts 10 years.
I agree with your larger point, but FWIW your example does not match my experience.
I've never had any special difficulties hiring Ruby/Rails devs. Quality is high, availability is reasonable (hiring is always something of a struggle!). My first Rails hire was in 2003 (rails-0.8, IIRC) and my most recent was about 6 months ago.
That's 17 years, and counting.
OTOH, I've never had a good time hiring people to maintain EOLed apps. Ruby or otherwise. "Sustaining Engineering" is a (misnamed) trade that isn't as broadly attractive as new development.