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.
I too get a lot of Rails spam from recruiters which I find hard to fathom since Rails usage is on the decline. What's your take on this? Is it that Rails shops will only settle for senior developers?