Implementing any kind of DRM is the game developer’s prerogative.

Yes, sure, but Steam encourages the use of the DRM included in the Steamworks API.

If Minecraft could only be downloaded through Steam, that would also be a step back insofar that right now, Mojang will let you download the game without any authentication at all. Changing this would be quite inconvenient for mod developers.

Allowing other versions is possible, again, at the developer’s convenience.

How? With branches / betas? There are a limit to those, and I’m pretty sure Minecraft exceeds them already. And as you already mentioned, you couldn’t use multiple versions at the same time. The only alternative (that I know of), the download_depot method, is far from user-friendly.

No other launcher needed besides Steam (and Steam is very far from being only a launcher)

That would reduce the number of launchers required from 1 to… 1. 🙃

Steam Workshop

The workshop is integrated into Steam and can only be used there (can’t download the files manually, as far as I know). It’s also way too simple for the modding needs Minecraft has (with its multiple versions, dependencies or breakage between mods, modpacks…).

It would also require Mojang to write their own mod loader, which would likely be too limiting for most mod developers. (And Forge is not the only mod loader out there. There are far better ones, like Fabric.)

There’s just no point to doing this. You’d replace a system that works very well with one that doesn’t. Steam generally works pretty well for managing games, but I don’t believe there would be any improvement if Mojang were to switch to it for Minecraft.