Unless it has been taken out of effect, Stonehearth has a rule that sounds very similar. You are not allowed to take another mod and continue it without consent of the original author, irrelevant of what the original mod was licensed under.
“Never will” is a bit a harsh word and I would dare say it could even be a developer issue: If you provide a tag for workshop items (i.e. “launcher”), the game could then enforce that only one launcher is loaded and otherwise throw an error at your head. In general though, Steam caters the general audience, that is true. The question is whether it’s a dead product by Valve (as in, left in the rain like so many other things), or if it’s being continuously developed to maybe amend for such things.
1 is technically not completely correct. For GMod, servers require addons too (of course) and can download them by providing a collection on the Steam Workshop - no account required. The download is done using an anonymous account on Steam (which means only public addons are possible, of course). This is not for end-users though. Plus I’ve also mentioned, uniquely identifying your users can also lead to neater telemetry and insights, something that other repositories can’t provide (because an account there is free and not tied to the game purchase at all).
At the end of the day, Steam is a store and as such it makes sense that they do not provide a repository for the whole world. As comparison, Twitch requires me to use a stupid account too, yet that doesn’t really count as DRM. We can argue whether the mere availability of non-account downloads turns the whole thing into a non-account situation, but for the sake of this conversation I would like to compare apples with apples and therefore go the automated update/one-click installation route (rather than “download a zip and DIY”)
2 I don’t know. I suppose a lot is left to the developer, because Steam offers more of a workshop than a one-stop solution. Personally, I never really had many issues with GMod six-or-so years ago, errors were somewhat easily logged or reported and the whole thing was a very smooth experience. Publishing tools were a bit rough, but after making a batch file/Perl script that wasn’t that bad either…
About the whole Skyrim debacle - I would like to think that this was more of Bethesda’s doing than Steam’s (seeing as they are now doing the exactly same thing on their own). The idea itself wasn’t bad and I would definitely consider making paid mods - I think the time wasn’t ready for it. Paid mods are a thing, it’s just that many take free mods for granted these days.
Setting up an own Discourse is more cumbersome (and expensive) than using something provided to you though. I haven’t extensively used the Steam discussion boards, but they’re better than nothing and definitely better than mere comments.
Git, not GitHub. git is a VCS, GitHub is a hoster. Nitpicking, but it’s an important difference.
It’s not “time consuming” at all, it’s not even difficult, there are libraries to control git (libgit2 I think?) that can be used and streamline most processes. However, git isn’t a simple protocol and it allows for many shenanigans - I would advise against it. On one hand, many of the features git offers won’t ever be needed - you don’t need a local history of the mod, nor do you need anything but the most recent version. On the other hand, since you’re pulling everything into a local workcopy (I assume), it also means that I can just put a FreeStuff.exe into my repository and easily ship that to everyone who has my mod - the control over what’s being downloaded is completely lost.
Plus, you have to (technically) deal with trusted hosts, credentials and all that jazz - it’s not exactly simple. In my opinion, it’s a complete overkill (and also quite proprietary).
A HTTP client on the other hand is relatively sleek to implement (… assuming there’s decent HTTP libraries out there) and they can “shift off” the whole work to another party. I might try to get one into the game myself.