One part of the fun of modding Skyrim was trying out all kinds of mixes, mod variants, troubleshooting compatibility issues, basically shaping the game on your personal whims, using the mod library as this great toolset to insert and remove whatever your pleased. I spent an entire day doing so, and it was lots of fun to explore all the different options and creations people had made available.
If, let's say, one third of such content is behind a paywall because these modders don't want 'entitled people' enjoying their content for some reason, how the hell am I going to replicate that feeling of shaping a personalized version of Skyrim, or whatever game? How can I mix and match if I first have to hand over some coin, no matter how small the value, just to access some content? And there isn't even quality control involved, like a genuine commercial product would be subjected to.
Mods being free to all was a great thing. It was an inherent part of the appeal to me. Free flow of creativity in all directions. It made PC gaming great. DOOM still has content produced for it because of that dynamic.
Sad to see some modders would rather have their products evolve to some kind of third party DLC. I'm sure they'll be successful, financially. But the dynamic will never be the same.