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?