It's a difficult metric, that one. In part because I get the impression that high-specificity tech like Elixir and to a certain degree Clojure attract people who, once they're in the job, _never leave_, and their productivity is high enough (if you believe the marketing) that you just don't need to grow a team as hard and as fast as you would on a more "popular" stack to get equivalent outcomes.
If all that's true, it's a natural consequence that better tools will have fewer job postings: you need fewer people to get the job done, and the ones you've got have really good retention. However, so will tools on the other side of the bell curve: nobody in their right mind wants to go there, either on the supply or demand side. So all popularity really tells you is where the enterprise hype machine was, five, ten, fifteen years ago such that "nobody got fired for choosing X" and "learn X! It's got a lot of job postings!" still align.
But the enterprise hype machine is optimised for tools that have a very specific set of surface criteria: enormous libraries so you're not inventing anything, rock-solid vendor support so there are people being paid to evangelise it into orgs (or, like JS, are ubiquitous anyway, but I think that particular lightning only strikes once), and "easy to learn" ergonomics such that developers asymptote towards replaceable cogs in both quality and quantity.
"Well respected but obscure" describes a certain sort of success, by that reasoning. It just needs enough of a community to stay alive, and that's a bit more of a toss-up. Ruby haemorrhaged people in the Rails 4-5-6 cycle as people jumped ship to node, but Rails 7 and Ruby 3 are looking extremely tasty despite that, and I wouldn't be surprised if we saw a bit of a resurgence in popularity over the next couple of years outside big enterprises.
Most of the people I interview have got into tech after Rails 3 was released, and generally aren't aware of why the Rails/Phoenix/Django concept was such a breath of fresh air. I think what you'll see is that the reason Phoenix and Elixir have a "next big thing" isn't technical factors at all. It'll be a rediscovery by the new generation when someone unexpected goes unicorn and the story is "it was all down to Phoenix", even when the real factors were elsewhere.