Is it the alternative?
For desktop or server?
NixOS will continue to be niche and continue to be an alternative.
For desktop, NixOS will continue to be a niche, like Arch and Gentoo, because of the learning curve. But if you're already running Arch Linux, NixOS is a step in a direction you do want to consider, but may not want to afford with your time.
For server, NixOS will also continue to be a niche. Using it both attracts talent and limits the talent pool, because lots of very capable DevOps and sysadmins just never got around to NixOS, even if they are veterans in both the classic and the cloud world. It's like making a startup with Haskell.
Also, I've been holding back on switching to NixOS, but I'm finally dedicating my next free weekend to install it on my laptop. I would love to be able to reproduce my exact working environment on new machines more easily. I'm not switching from Debian / Alpine on server any time soon.