If you have a working system and no wish to ever install anything new, you could run it indefinitely.
It wouldn’t get any updates after some point and after ~10 years some websites would stop working because they would be using some new standard that is not yet implemented in the browser on your machine.
To update to a newer version of NixOS, you might need to change config slightly, and that requires you to know where configuration is and how to read error messages.
To install something new it is easy in 70% of cases and really really hard in the remaining 30%.
In my case, it just worked, on two machines. Just added steam and lutris into the config along with two other options recommended for gaming and it worked.