Syntax similarity probably reduces the "fear of new" (different) that prevents some people from making the transition. But the biggest factor is that the creator of Elixir, José Valim, is a well respected Rails contributor.
But indeed, Elixir is so different from Ruby that one doesn't just casually move to it from Ruby without learning to think functionally.
To your question about Rails, I'm not sure I can say whether it's in decline or not. But I do know that many hiring managers are not technical enough to know what's important, so they try to replace exiting employees with people who have the same skills. And probably there are one or more Rails apps that still need dev/maintenance. As far as money goes, Rails is still a very lucrative career focus, at least in the US.
Rails adoption in the U.S. relative to it competitors - Laravel, Node and Django - is much higher than anywhere else. Outside London Rails adoption is very low in Europe.
But indeed, Elixir is so different from Ruby that one doesn't just casually move to it from Ruby without learning to think functionally.
To your question about Rails, I'm not sure I can say whether it's in decline or not. But I do know that many hiring managers are not technical enough to know what's important, so they try to replace exiting employees with people who have the same skills. And probably there are one or more Rails apps that still need dev/maintenance. As far as money goes, Rails is still a very lucrative career focus, at least in the US.