The "Import from Zip" is all fully manual, and you could do that with Twitch as well. You could fully flesh out your own modpack manually, and then export to CurseForge. You'd then have access to all the version controlling for all the mods you'd chosen to use in your modpack integrated into the pack as standard. You could "Import from zip", just by unzipping the pack into the profile folder, then clicking update. The manual stuff isn't the issue. It's the lack of one-click-installs.
If VS created a bespoke mod manager and launcher that worked very much like Twitch, with version control for individual mods, as well as modpacks, in-built search functions, with one-click installs for mods, modpacks, resource packs, and moving forwards, different shaders, skins, and pre-genned maps, it'd be good. You'd also need to be able to use multiple instances of the game as standard, be able to use multiple accounts as standard, and be able to seamlessly use different versions of the game in each instance, without it asking to uninstall other versions.
To me, creating a MultiMC-esque clone would be largely pointless. So much of it is manual that we might as well just copy zip files into folders, and call it all done. Creating a Twitch Launcher clone, however, would add huge playability to the game. You shouldn't need to manually download any file, open any browser window, or open Windows Explorer at all, ever, to be able to mod the game, and run multiple instances, over multiple versions, on multiple accounts.
Comparing to MC isn't fair, in this case. There's a lot of variables there, and it is all run by the modding community, not by Mojang. Different modding groups tearing the community is different ways. Up until 1.12.2, it was largely harmonious, with Linux lagging behind, doing their own thing. After 1.13, it all changed, but that was nothing to do with Twitch, or with MultiMC, it was all down to the developers of the modloaders. With VS, modloading is fully integrated into the game as standard, so these deviations wouldn't affect it. Do it right, once, and it only ever needs to be refined, not overhauled. It's a perfect fit for the Twitch Launcher, and creating anything bespoke would only ever be an imitation of what Twitch and CurseForge currently offer.