"Nowadays I keep finding RoR jobs but I see more and more legacy applications that need to be maintained in order to move to a new technology."

This is exactly the situation with a number of long-standing, formerly popular languages. ColdFusion and PHP jump to mind. They're fine languages and people love them, but if no one's paying, why bother sticking to them? Loving a programming language doesn't pay the bills.

"I'm sure it will keep providing as long as Ruby engineers become founders or companies keep maintaining legacy projects built with the technology."

My previous employer was maintaining over 30 acquired company's worth of legacy code bases. They replaced common functionality via new microservices and would sometimes replace an entire product via a new acquisition. It's sometimes a better ROI to replace instead of maintaining or building.