Again, the repercussions will be much bigger than merely some mods being free and others paying. If you can't see what the inclusion of monetary competition and reward is going to do to modding communities then I don't know what to tell you. It shows a glaring disregard for human nature.
The move is very self-interested on Valve's part. Given the ridiculous cut which would turn modders into freelance developpers creating any content they might need, it seems they've practically given up on developping games at all. Source 2's main focus is on "content creation", updating the Hammer tools into something more user friendly and comprehensive. This deal with Bestheda is to prepare their customers for the inevitability of Valve getting their cut on literally any mod worth playing developped on Source 2. Tbh...if it's modder friendly enough that amazing games can be created and the content creators rewarded for it, it may end up as a positive all around, for customers, modders and Valve alike(although more positive for Valve than anyone else, obviously). I'm not wholly willing to burn Gaben's lardy effigy in anger just yet. Hearing about the willy fat bastard's team overall strategy with Source 2 has made me slightly more optimistic about this move. But a company like Bestheda? That releases buggy fucking games aimed at the console market and expects the modding community to fix it for PC? Whose modding communities are responsible for a huge amount of content, publicity and sales? Retroactively applying paying mods to a game that has been around for years and has already made them a buttload of cash? Fuck them. The less scrupulous and greedy companies are going to make a killing out of it if they make similar deals with Valve, like they do with dlc, and early access, and all the other recent marketing gimmicks which add zero value for the customer, merely takes content that would be considered as part of the complete game as a matter of course one iteration back and parcels it behind paywalls. It's all inevitable really, as long as people keep paying more for less, and they undoubtedly will if recent history is any guide.