I've never seen a ten year old Rails application, but from the Rails applications I have seen, I'm horrified by the thought of how it would look after that long.
I've been in a company that let their Rails applications grow for 7 years. Believe me, the oldest code was not pretty to read and almost impossible to refactor. When we finally had a real frontend team, the only way out was to reimplement the apps with a new architecture.
At one point I realized, I should not stick into one language and one framework. Ruby is good to know, but I'm better if I can also tackle Scala, Java, Rust, OCaml, Haskell, Python and so on. Also happier, I'd say... I can now choose the best tool for the job.
