Niche for your, but as I stated above, it is gaining support more and more.
https://www.directv.com/technology/4k
I don't know, direct TV is kinda small though LOL. Yes no broadcast TV, and limited options in 4k, but in a world where broadcast TV is dying in favor of streaming services that do 4k, I would say it is a little more than niche.
- Netflix user base is growing, at least according to their latest financial results and is putting on some quality programs I watch
- Stranger Things
- Daredevil
- Ozarks
etc. Hulu is having some success in this arena as well as amazon. You get the idea, IMO it is going beyond Niche.
It's not full mainstream, but for a niche product, everyone has an upgrade path to it
http://4k.com/news/meet-comcasts-new-x1-4k-uhd-hdr-dvr-box-its-finally-coming-19842/
If Netflix, Hulu, Amazon, Vudu, all support it, it does reach a rather large audience