story
So yes, "re-train". Give everyone a book on the new language, maybe pay for some online courses from pluralsight or wherever, cancel meetings for a week. You can learn a lot faster than in a school environment when you've got paid 8 hour days to put into a single subject + coworkers to chat with.
Besides, it's not like they don't get to avoid learning new things anyway, even if you restrict it to the Ruby ecosystem. In the JS world (which I'm sure they all know too, as one tends to when working on web sites even if you're mostly back-end) as new revisions of the language come out people have to keep up with the syntax and changing idioms.
"For some reason, programmers love to learn new stuff, as long as it's not syntax." -- Steve Yegge
Mandelbrot (from CLBG) Ruby 246s NodeJS 8s Java 4s
Web (fortunes from TE benchmarks) Ruby + Roda + Sequel 51k rps NodeJS + Express 46k rps Java + Dropwizard 62k rps
Java (vertx-postgres) 347k rps, Go (fasthttp) 320k rps Rust (actix-postgres) 607k rps
fasthttp isn't even a web framework. It's not surprising that using a raw HTTP library is dramatically faster than using a full framework and ORM but it's also not a sustainable way to build complex web applications with 1000+ developers.
I can't speak to fasthttp as I haven't used Go much, but actix-web in Rust is a full framework (not as full as something like Rails, but certainly more than mature enough to be used for production projects).
