Especially when Steam Workshop is right over there, offering prime discoverability, easy installation, automatic updates, and feedback tools for mod authors.
Steam workshop is not a boon for modders, it is a veritable nightmare. It does not allow you to have different mod versions in 1 subscription, it does not enforce game versions properly AT ALL, it does not care about mod relations. And most importantly, it has VERY absurd restrictions when it comes to the kind of mods you can upload. And I am talking technical stuff, not content stuff. If your mod adds just pack files to a game then nothing wrong with that, but if you have interdependence, external DLL side-loading and stuff like that (think KSP) then it's a total nightmare. And lastly automated updates BREAK all mods silently. Steam does not even know which version the game is on. And I mean that literally, the workshop does not know your game version, it only assumes the "latest"
TL;DR
Steamworkshop can not, and should not, ever, be the only way to get mods for your game. You'd have to design your game with steamworkshop in mind if you wanted to support all "real" mods. Kinda like Skylines
That's all fair, and given that they want non-Steam versions of the game, it shouldn't be the only way to get mods. Absolutely.
But as a user? One of these usage scenarios is vastly better than the other:
1. Find mod in Workshop. Click install button. Play. Hope that mod author keeps it updated if a game patch breaks it.
2. Find mod on forum. Download it. Figure out where to unzip it, or *how* to unzip it for less experienced folks. Play. Occasionally check forum to see if it's ever updated, or forget to do that until it breaks, then try to remember where I got it from to go do that again.
Given that we're talking about XML files for modding here, I don't think we have to worry about content restrictions a whole lot. Plus, doesn't Cities have compiled .dll mods along with the asset editor? So it seems like it is doable, although I'm not sure what KSP is doing.