UWP doesn't scream good things for the future of Windows gaming, but I'm glad they did it now with a backlash, rather than later with Steam in ownership (for then the damage would have been irreparable). There's little that can be done over their dictatorship considering they own Windows, but we can hope this outcry hit some sense into them and they don't try to run off like that for the foreseeable future.
Cross-platform "shouldn't" reduce mod support. Developers with interest in mod support will continue to offer it regardless (as they already have been doing). The communication between Valve and MS would open a door for cross-platform rather than create a hurdle for developers. Many who had the option for cross-platform chose against it for balance of Mouse&Keyboard vs Controllers (GTA5, R6S and Fortnite come to mind). Single player games would be untouched completely and personal server multiplayer wouldn't be hit hard either (instead having modded and mod-less servers, restricting console players to not be able to join unless the mod has been ported to the consoles).
I agree that monopolies are bad, and hope MS don't claim too much power for the community to handle, but I still believe good would come from this rather than bad. It would improve connections and offer more tools/options to developers (albeit all under the same companies ownership/pricing), which would accelerate the growth and development of video games. This isn't a situation to climb up on the high horse of "PC Master Race" over. It's not like MS are going to touch your current library either (there isn't a company in the world with enough funds or processing power to change all of steams games so they run in a UWP format).
WORST case scenario is MS decide to set rules that make future products on Steam restrictive. Should that happen, another company will step up and offer to carry the public. Be it GOG, Humble, Blizzard, Origin or Tencent; companies would start fighting over creating a new strong storefront to become king of the PC marketplace. Microsoft wouldn't want that to happen after spending so much on Valve, so I very much doubt anything notable will happen within the first few years of ownership. As I said before, I wouldn't even expect change to happen with the PS5 and XB2's release.
I still stand by what I said on principle alone on what that trashcan company has done to this hobby I would instantly go to the Steam Discussion forums and post as many piracy links I could find effectively nuking my account. I have no desire to crossplay with the casual Xbone trash that continues to support "games as a service" nor do I desire to have consolized gaming on my PC. You can say it's elitist cause it is. Having put more time and understanding in this hobby to have it constantly dumbed down to the lowest common denominator (Xbone fanbase) gets more irritating every year. I thank god that Japan still has developers who actually treat the hobby as the art form it once was. All I see in most Western Devs like M$ is Less than 2 year developed pay to win trash because they don't have the talent to create something that requires actual effort. I support Steam because they let me mod my games. Moving forward into UWP would absolutely disgust me and M$ would do it given they could get away with it cause it's a fucking MONOPOLY. Anyways that's my rant take it or leave it. I despise that company. What they did to Rare is a perfect example. Don't bring up Sea of Thieves btw lol it's such trash. "WORST case scenario is MS decide to set rules that make future products on Steam restrictive. " This isn't the worst case btw it is the case. This company knows fuck all about gaming with Phil "Don Mattrick 2.0" knowing nothing about actual gaming. Jeez man like disgusting the though.
Cross platform I want none of as it takes away mods. I also have no desire to play with console kiddies. On PC you have "some adults" which is debatable because you have to know what you are doing. I enjoy custom maps in my games and cross play takes that away for the console peasants. (Yes, I used that term don't make a big deal out of it who gives a fuck) I'm in the midst of freaking out right now because if Micro$oft does somehow pay Valve I'll have to nuke my Steam account because it basically means they are going to destroy the platform along with everything else they touch. Yeah love that unmoddable UWP trash API. I own licenses but at least the licenses that I own allow me to mod the product I paid for.