Gede: MS is struggling to lock users to their platform. Many have left for Google or Apple (not for Linux or BSD, sadly). This is MS's final (or near final) fight to remain relevant in a changing world. Valve caught wind of this effort and left a door open for dissidents to move to Linux. That was quite wise and clever of them.
Yeah, they're clever by opening a system that's bad for Linux? If anyone wanted to move over to Linux, SteamOS is their last choice.
I would also like to take to opportunity to reinforce some of the concerns that were expressed here: developers can break or make UWA. I'm guessing they will break them in the first years, not taking the time to adapt them to each platform. "Write once, run anywhere" is a myth for interactive applications. For each additional platform, you will have to revise all the user interface, or risk negative reviews from users trying to run CorelDraw on their XBox. Yes, they can make the app run in several platforms.
I guess we all haven't see how Universal Apps work until now and we're just making assumptions. Universal Apps, while their point is really what you said it is, code once and run on all, you can simply limit these apps to one ecosystem or another. For example, suppose you develop an Universal App. You can develop it and make it available for all. Or just Windows and Windows Phone. Or Windows Phone and Xbox. Or just Windows. And since they're making apps that run on all devices natively and without issue, then MS probably took that into account with their development tools and shows you how they all act. What is it a 'myth' for? The only real difference between all of these platforms is the user interface and how it receives input.
Real gamers won't stop using Steam and GOG. Real Pros know what they want and purchase their applications from the developers, who want to avoid the big (unneeded until now) MS cut. Who is left? The casual PC users. They are many, but they won't spend much. Will they be able to keep the platform afloat until it reaches maturity and profitability? And how long will it be that way?
Ha, real gamers won't stop using Steam. First of all, there's no such thing as
real gamers, regardless of where they buy. Secondly, Steam is always the worst alternative.
As for the second thing, again, that's for the future to decide. With Windows 10's growing market share, it's only a short time before we see for ourselves; and even if the returns for the Windows Store weren't much, do you really think they'll quit it? Windows OS sales are dwindling, even makes up a measly 1% of their stats as far as I'm concerned, yet they continue releasing the operating system complete with updates.
I can see some people benefiting from this initiative, and UWA has some potential. But personally I don't like it, I won't use it and I won't recommend it to anyone, simply because it leads to the "walled gardens". Microsoft wants to be Apple, only its followers are not as loyal and have more alternatives.
As has been said, you can sideload your Universal Apps, bypassing Microsoft's 'walled garden' completely.
Some of these complaints may need some time to sort themselves out. Isn't it too early to complain about everything that seems to be decided in the future?