Sure, just like SkyUI is "optional" for Skyrim.

Sure, you can. But you will gouge your eyes out.

But Cities Skylines 1 is borderline unplayable outside of Steam because the non-steam players can’t use that one third-party traffic mod on the Steam Workshop that fixes the annoying only-one-lane traffic jams the devs did jack shit about until their recently-released sequel

100% true - but if people feel the need to create so many mods, then there are probably lots of things people feel aren't good enough about the game. I'll admit my gaming time is limited, so just researching and adding mods could easily take all my time. I mean, fuck, I sold my Warthog HOTAS and went back to a cheap thrusmaster not because I liked the thrustmaster better, but because I was spending more time writing and fixing scripts and updating my bindings than actually playing the game. And every time an update would come out that would break a script I would spend pretty much my entire gaming time budget for a couple weeks just getting it running again. It got to the point where I just didn't play those games because every patch would change something and something (even something small) would break or be incompatible. I'm kind of over that.