Used to be a modder, for what it's worth. Mostly for NWN but also some other smaller games and mods here and there. Stopped for various reasons.

One way or another, I can't say I'm surprised this happened. This has been a long time coming. Well perhaps not Valves implementation that seems a bit rushed in light of the UE4 Mod Thing. But the idea of mods as paid content, that one has been simmering for at least a decade. I can't begrudge people that think their work should be paid. And it's not like this doesn't potentially open up opportunities for really talented people to perhaps even live of their work. It really does.

But what it also opens up is a can of worms. At the best of times, modding was a place of low profile jolly cooperation and high profile Drama with a capital D - for Dick Waving Contest. Egos frequently clashed, ideas and works were nicked and rebranded, collaborations turned sour as the involved parties developed different visions for a project.
And the users weren't much better. They'd form cliques around their favorite modders and do all the stuff that internet groups tend to do when they get really zealous about something.

And into this very scenario of volatile, flammable agents, Valve tossed a lit match and said Cheerio. I can't imagine doing collaborative work under this new system. I know from experience that even without the incentive of money, friends can and do backstab for the most minor of reasons. Now they can do it for money. This is a shitstorm. It will remain a shitstorm and the shitstorm will result in a lot of quarreling over mod rights and probably several legal cases.
I can see it causing tremendous paranoia within collaborations - which used to be a center piece of modding. It's usually not just one work but a long chain of various mods at work. Now at any point of the chain, someone can decide "Yeeeap. Money now" and profit of the work of others, as the chain depends on the interlinked parts.

Maybe it'll turn out right. But I can't see that. Not with the way Drama unfolded before. Now that Drama has the added fuel of money to it. What a mess.