Of course this was going to get PC gamer outrage!
The one thing PC gamers have always had over consoles is free-mods. Mods can really expand the lifespan of a game-title. Mods have often fixed broken things, added new content (assets, maps, game areas, new NPC's, etc).
Could we imagine how not-so-well Dark Souls 1 PC would've sold, if Durante's DSFix was a $-Mod? Would everybody buy that $-Mod just to run DS1 PC above 720p? How much would Durante have charged for it? Would Dark Souls 2 PC even got a PC version or a proper PC port by From Software, if it wasn't for what happened w/ DSFix being free?
I could imagine how us PC gamers would feel, if any of these things did actually cost money: DSFix; DPFix (for Deadly Premonition PC); any of Wesp's unofficial patches/mods for Vampire Bloodlines; any of DrogBlackTooth's unofficial patches for Arcanum; any unofficial texture mods for System Shock 2; any unofficial texture mods for Thief 1+2; etc etc. Just think about all of that, for a moment - let that sink in.
How would other old-school MP-focused games have turned out, if mods weren't free and they came out in this new $-Mod era? Remember when CTF modes (before every dev put it in their game) and Freeze Tags modes were really free-mods? What about new maps, which in the old-school SDK era (before official pay-DLC came out) were what helped games like Q2, Q3, UT, and RTCW keep their life-span alive for a long time frame? That's right - these games wouldn't have lasted as long as they did, if it wasn't for SDK's and free-mods. Putting mods behind $-wall, that's obviously going to make less people use your mod.
Look at games like Titanfall PC, where you feel like after 3 hours of play, there's not much more official content to really get. How crummy would it be if a game like that lacks official content and the mod-content you might want is behind a $-wall b/c mods are paid for. Who the heck is going to spend even more $ just to get more 3rd party mods and content?
It's already a pain-in-the-ass trying to keep up w/ official content from the base-game dev's - i.e. $-DLC's + Season Passes. It's even worse when games have copious amounts of pointless DLC - and for some reason, you might want it all. This is especially true if you're trying to Collect Everything for a certain game...even more so true, if they don't later release a Complete Edition with everything! If $-mods come out, we'll NEVER be able to get a Complete Edition for a game we love for free.
Sure, it'd be really nice for deserving modders to have their hard work and effort reward with $$ - especially if they made their own assets for it. But, the problems of modders stealing others' content and selling it as their own is also going to be a problem, too. And well, the floodgates are already open. If everybody wants to charge for their mod, then how long will a game's lifespan even last? How many companies will make a game, port it very basic-style to the PC (i.e. see Dark Souls 1 w/ lack of PC features and locked in 720p), and then let modders charge for the UI improvements and other things (i.e. see DSFix, if Durante decided to charge for it)?
Another thing - what about if the original dev updates their game and it actually breaks a mod you paid for? So, we already known Steam ain't too friendly for allowing gamers to run older-versions of game - especially if an update is forced, even if you select the option to not have a game updated. Well, until modder fixes your mod - you're screwed. And with the way this is, this is the Wild Wild West right now w/ mods - so, it's not like the base-game dev's or the publisher have control and be very selective over what mods they will allow; or even they can go ahead and make sure certain mods they selected work w/ their particular game.