They still add updates to the game once or twice a year, so you usually want some of the new features. That can also lead to the issue of when you finally do decide to update 3 years later or whatever, now it's a 4 hour ordeal to fix instead of 30 minutes. There's no really perfect solution, but it's worth it if you love city builders.

Steam updates the game and mods automatically. It can be overridden but that's a pain in the behind.

Can it ? In my experience, once Steam is aware that there is an update, you cannot even start the game before updating, even in offline mode... but that was some years ago, maybe this behaviour has changed ?

(Don't use Steam.)

Steam has the option to let you pin a specific version of the game if the publisher supports it, but Cities Skylines does not.

Yes, that is an annoyance. I think that Steam should put more pressure on developers to make old versions of their games available. I am glad that the software support is there (even if the UI is buried; it was originally intended to support beta–testing programs, complete with passwords to unlock the beta releases), and that developers sometimes use it, but I wish it was a formal requirement for publishing on Steam.