I expect to see rails slowly die off over the next decade, with many rails developers moving to either Elixir-based web framework Phoenix or something written in Crystal. There are a few web frameworks in Crystal, but none are fully complete yet.
- http://kemalcr.com
- http://luckyframework.org
- http://amberframework.org
Swift will be more prevalent than Crystal. There are already some serious frameworks available and you get the additional bonus of being able to write Mac and iOS with the language as wel.
The server-side swift ecosystem is incredibly small, and that's by far the most important thing for adoption.
You're not really going to share backend source with your iOS apps, so there's nothing to really gain there. Languages are so homogenized these days that moving between the popular few is pretty simple and requires fairly minimal rampup.