Neither are "next languages," all three differ significantly in scope. For the western Ruby dev (meaning Rails people) there is a much clearer and more consistent path forward with Elixir/Phoenix.
After a couple of paid projects with Phoenix I'm not sure it's a path forward. More like a fork in the road to a different destination. I'll steal pattern matching from Elixir and keep using Ruby on Rails. Phoenix adds too much unnecessary complications for the average web app, starting with Ecto. Potentially great when building complex systems, but there are not many of them.
My suggestions are: simple to average web apps, stay with Ruby (Rails) or Python (Django); complex, look into Elixir (Phoenix.)
I never used Crystal so I can't say anything about it.
Would you care to elaborate on the complications you've experienced?
I'm curious because my personal experience has been very much the opposite. I've built nothing but small projects using Phoenix, and I've considered it to be a very straightforward companion whenever I've needed to add a web interface to my Elixir apps.
My experience is that it's not so much that Elixir and Phoenix are complicated, but rather that they're very different from what a Rails (or Rails-like) developer is used to.
There's not just Phoenix' deceptively Rails-ish approach, but also Elixir as a functional (and somewhat strange?) language, and while you can do without knowing much about OTP, it's not exactly invisible.
On top of that, I find that all the best packages embrace this weirdness. Ecto is wonderful, and by far my favorite 'ORM', but it's very different from the more common ORM's. splitting up the view layer in templates and views is, in my opinion, a step up from the Rails approach. But it took me a while to adjust. The sparing use of macros is also 'weird', but again I find it a step up from what came before.
I kind of feel the same way about Elixir/Phoenix as I do about functional programming in general: Whether it's actually as complicated as it is often said to be or not (perhaps just a matter of 'what we learned first'?), I find that once I got over the hurdle of learning the building blocks and mindset, the stuff I could do felt more fun, more expressive and less brittle, and the approach is starting to infect all the other coding I do.
At the same time I completely understand that if you're comfortable with Rails, and it does the job, or if you use a lot of gems that might not have a (mature) equivalent in Elixir, there's no good reason to bother unless you need channels, perhaps. I have the luxury of being able to use my own tech for many projects, as well as the time available to dive into the Elixir/Phoenix ecosystem.
My suggestions are: simple to average web apps, stay with Ruby (Rails) or Python (Django); complex, look into Elixir (Phoenix.)
I never used Crystal so I can't say anything about it.