Yes, it's old. Yes, there's only little interest. Yes, there are _still_ many technical problems.
The interesting thing here is that technical problems can be solved, and will be solved given enough time. Heck, I've heard rumors that even Windows does not crash multiple times a day anymore.
Focussing and working on what is popular will most likely create a world that is less free every year. Some people rather work on something that seems right and promises a better, freer future. Even if it's not popular (right now), even if it has a high probability of failure.
Popularity is a very volatile property...
And popularity might be a double edged sword, but double edged swords are still useful if you're careful.
Although keeping software proprietary or writing proprietary software may have become somewhat unpopular in some niches (I'm not sure it's incredible unpopular seeing how many people and institutions still use and often advocate Apple, MicroSoft, use FaceBook, WhatsApp, whatsnot...), I'm not yet convinced that many people (say more than 1%) really believe that all software should be free and act on that?
Out of all of the microkernels out there, the one I like the most is SEL4, but the security model is so different from most OS kernels that you essentially have to program for it from the ground up. And it seems like there isn’t any momentum there either. QNX is already very mature and performant though, and it has gotten a huge boost from IoT and electric car development, and IMO all it needs is an open source license and it will eventually creep its way into general purpose OS usage.