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With NixOS i'll leave it for a week and go to another distro and then I think to myself "what am I doing? I can just do all this on Nix" and then I go back to NixOS.
On my NixOS I have a few destroboxes set up. One for Arch, one for Fedora, Debian, and Void. I use all of them for various things so I NEED to know all the quirks of various distros. So the knowledge I've acquired from the various Linux distros never goes away, I still use it all daily. Also, you're right, your fear is valid not so much in a way you'll potentially "lose" that knowledge but rather...you might get bored.
Arch bores me now. it's too easy. NixOS is just so much more interesting and fun to tinker with. Like for example last night I put Arch on another machine to tinker with and already i'm looking to uninstall it. It's boring. Nix has also spoiled me with knowing EXACTLY what's on my system and having it all right there in front of my face. I enjoy version controlling it, tweaking it, making my configs cleaner, more effeciant. I don't feel I get that on other distros. Sure there are tools to mimic the "feeling" of NixOS on other distros like The Black Don's Dcli for Arch but it's not the same and even he'll admit it's not the same and just end up back on NixOS. But now I'm just rantnig.
If you're concerned about losing that knowledge of other distros do what I do, just set up a few distroboxes and use them for various applications. like for me Tabby isn't packaged in NixOS and it's been several months since the last person made an update on said package so I simply use my Arch distrobox to use it. There are some really niche .deb programs that either don't work well on NixOS or don't exist so I use my Debian distrobox. On the opposite end there's stuff like Supersonic which actually works much better on NixOS than Arch so that's an obvious choice. mix and match. build out a complete Linux Box with NixOS at it's core. once you do...man everything becomes a breeze.