Is Java Really Losing Popularity Among Developers?

Doesn't seem to be losing popularity among employers.

The majority of jobs with newer companies or even newer projects at larger companies tend to involve C#, at least in my city.

Every day

There's absolutely nothing stopping any developer from learning other skills and changing jobs. I was an enterprise dev that taught myself Ruby in the evenings until I had enough skill to move away from the enterprise world. I've been making a living from Ruby coding for just over 3 years now (probably worth noting that I'm almost 40 and have 4 kids). Pay is slightly less, but work satisfaction is so much higher is't a joke.

Seriously, your current employer might not let you change, but you don't have to stay with them forever. It's your life, not theirs

[Edited to add: I see comment here about job ads. What I discovered early on is that in the Ruby world, and probably Python et al, most jobs are got via reputation or mailing lists. pretty much never on mainstream job boards]

In some cases, I've seen jobs change their requirements some, simply because they can't find people when their requirements are tied to one specific tech. Not often, but more than I'd have expected.

Overall trend graphs may show one picture, but available people with required skills in a particular geographic area would show large differences in some areas (educated guess, at best, but seems to bear out via anecdotes from friends in my network).

The jobs that change tech requirements to expand the list of people that can use tend to be centered around smaller/shorter projects. A company that's been largely python by default for years may grudgingly open up their job search to php or ruby people, both in hopes that those people can pick up python, but also realize that they'll probably have new systems written in a new tech stack, and that's just the way it goes.

It would be easier for developers to 'go' where the jobs are if the 'going' was more able to be remote, but for many reasons that's still not a reality for the majority of tech workers.