Sharing happens in virtually any healthy modding community, really. Just as an example, I've just looked up a list of credits for an old version of my CKII mod VIET, right before it was integrated into the HIP confederation, and if I were to charge for it, I'd also have to consider
at least thirty eight other parties who should receive some share of the profit. And I say parties, because some of these aren't just individuals, but teams of individuals, which means each party could be two or five or twenty people alone. And that's not counting if said other parties took stuff from other modders, potentially making the number of parties involve increase exponentially. And not to mention some of these parties I took stuff from on an unofficial basis - ie they supplied a bit of code to me out of goodwill, or it was unreleased stuff, etc.
And this is for a version that came out more than a year and a half ago. In that time, more people would've undoubted been added to that list, and a few would've been taken out.
Oh, and to top it off, since Ordep was my official partner, we'd have to split VIET's share evenly in half if I wanted to be fair.
I don't know how Skyrim modding works, but I'm sure there would be similarities.
The biggest issue is what happens with very large TC type mods with components from a wide variety of sources. I can't think of anything from Skyrim off the top of my head but say it becomes standard policy for all workshop games like EU4. Than lets say something popular M&T becomes a pay for mod. It contains parts from a ton off different sources and suddenly we have a copyright hell problem where other modpacks with those parts could get taken down like VeF or the owners of the mod components themselves kill the big mods. This is a can of worms that will make all parties less trusting of each other in modding communities
Skyrim has a number of large overhaul mods comparable to M&T for EUIV (or CKII+ and HIP for CKII), and though I don't really care about them, I'm certain they contain quite a bit of work taken from others. I feel like overhauls aren't as popular in Skyrim as they are in PI's grand strategy games, though; but there are still plenty of other large-sized and mid-sized and probably even small mods that use a lot of assets taken from other modders.
Gigau is not planning to make M&T a pay-mod, as far as I know (even if he could).
Some of those tacky weapons included in the Debut pack appear to have been based on weapons appearing in Blizzard games, hence they carry the banal line "allowed by Valve". Even if that were true, how on earth could Valve allow such a thing?