In the past Bethesda refused mods to be sold because they had no way of controlling the money themselves. All the money would have gone to the modder, and none of it to them. But after moving their sole distribution to Steam (even if you bought a physical disc in a store, you still have to use Steam to install and play the game), they finally realized that they have a means of controlling sales, so they make the money, rather than the modder. So of course now they are tapping the previously untouched revenue stream of mods.
Skyrim is not the first game to have mods for pay. Valve started it with their own in-house created games like Team Fortress. The fact that they made a lot of money from it is what made Bethesda take notice, and I am sure a lot other game companies. Even though it is far from a new game anymore, Skyrim still has a huge number of players and a gigantic mod library. So it was an obvious choice for the first non-Valve game to go the paid-mod route. It is a win for Valve, and a win for Bethesda. Only the modders and players lose.
It is also worth noting that Bethesda does not call all of its own shots. Just like Bioware does not. They are just a subsidiary of a much larger investment corporation. That corporation has lost a lot of money lately. So they want to make it back any way possible, and selling mods is just one way the mega-corporation can do that.
And yes, Valve opening up the bandwidth for Skyrim mods a little while ago was definitely laying the groundwork for the paid modding. It was a necessary first step for the business. What I can see as a next step is to make Fallout 4's Geck (that is what they call the Fallout creation kit), to be more integrated with Steam, so that you cannot save your mods locally. Instead they will probably only allow saving to Steam's servers. That way Valve and Bethesda will have complete control of the mods, and cut out sites like the Nexus. They will claim it is a good thing, because online storage will protect your mod from being lost due to things like hard drive failures.
Will Valve start charging a monthly fee for Steam use? That I kind of doubt, since even single mmo games with monthly subscriptions usually fail. Very few people are willing to pay $20 a month just to play a game. So I don't believe people would pay Steam a monthly subscription. Instead they would just pirate all their games and cut Steam completely out of the picture. People who do not want to deal with Steam now - when it is free (albeit the internet connection is not) - have been playing pirated copies of Skyrim since the first day it came out.
Of course whether Valve is smart enough to understand that is another matter entirely though. Companies are remarkably stupid when it comes to DRM schemes and piracy. They never seem to realize that DRM does absolutely nothing to deter piracy. Instead it costs them more money to buy the DRM scheme in the first place, makes their game more unstable and sometimes unplayable, and insults paying customers by treating them like criminals. In the end DRM schemes promote piracy, because for many people it is literally the only way they can play the game because the DRM prevents them from doing it legally. Even the remarkable success of DRM-free distributors like Gog.com and donation sites like Kickstarter have not clued most of the gaming industry into the fact that you do not need such absolute control to make money. People will still buy games even when they could pirate them. Simply because we are honest, and want to reward the developer, and insure they continue to make good games.
This is the most informative fact filled post I've read yet on this whole situation; and the thought of them controlling the Geck right from the start - I never would have thought of that, but now that you've said it - oh yeah, I do see that happening. And I've been afraid all along that Nexus is about to crash and burn because Valve-Steam will sweep it out of the way.
Hugely important post you made here, SubRosa. Huge.