I don't think that dynamic is going to change. bethesda have not really shown any interest in trying to sell player mods, and it looks like featured mods will still be determined by user votes just like Nexus. The only real difference is where the files are stored and what features each service adds. I don't think they have any malicious intent, they're just trying to do something to make mods more accessible to Mr. average joe who never goes anywhere on the internet except to check his yahoo mail. Bethesda isn't some big bad corporation out to steal your content. They want the community to expand just as much as we do. Every satisfied player is pretty much a guaranteed sale of their next game, with likely word of mouth advertising to boot. Skyrim wouldn't even exist if it weren't for the cult following of Morrowind.
Pretty much this, I actually see the workshop as an very good think, all the time since Morrowind the creation set has always been an sort of free bonus for fans game who was given away because it was just a couple hours work to modify, package and release it.
It was close to not being released for Fallout simply as it was another game series.
With the workshop they takes this mainstreams and are pretty much committed, yes it will be popular, most buyers of Skyrim on PC don't use mods but now it get simple enough for everybody. Chances for no CK in TES 6 or Fallout 4 is very microscopic.
Why does they do this, 1) keep fans interested in the game longer, they helps selling it.2) anti piracy steam Workshop require an registered game, yes it's possible to download mod other places but it require some knowledge.