It is exclusively about money for Bethesda. You can tell by looking at the last time they implemented paid mods, where they took a 25% cut for doing nothing. They offered no quality control, no resources, and boy howdy were a lot of paid mods stolen content, but they didn’t care because they wanted that money.
As to the modders, the only offer Bethesda can give is a wider customer base, but the assumption that you will make more money offering your mod for a price isn’t founded. We will see a large amount of shitty mods clogging the store using asset flips to maximize returns, because that’s what makes big money on mobile right now. Mod quality isn’t going to be enhanced by this: Mods will remain the same. You will just more of the bad ones. $99 horse dicks, anyone?
Bethesda’s goal, as usual, is rent-seeking. They can’t penetrate more markets, so they need to make new ones, and what better way to do this then to hire what amounts to contractors doing gig work. They don’t even have to pay them except in commission, which is a really scummy thing to do.
Some people see this as a way for mod-makers to make money, but mod-makers already have those! Every mod I’ve seen and every modder I’ve talked to has a donation link you can send money to, and the ones who didn’t had organizations and charities you could send your money to instead.