No, everything that could bring more gamers to PC can't be that bad for Valve because most games will still be bought there.

But SteamOS is even more doomed now because it wanted to attract console gamers. The Xbox-App makes Win 10 more attractive for Xbox gamers.

Man that would suck big time, my Steam library has about 200 games on it now

However, it would be more natural to have games, like most software, in the OS's built-in store... it would also be great to control the fullscreen interface with the XB1 or XB360 controller! I would rarely need to pickup the keyboard with a setup like this!

Anyway, before they started Steam, Gabe spoke to MS and offered them to start a digital game store, with their vision they did not see the potential of the idea (which is strange because this was the time this kind of central repositories with search etc. started to appear in Linux distributions and after trying Ubuntu back then, or a little later, it felt like this was something sourly missing from commercial OSes).

I'm really surprised they haven't worked with Valve to have cross-buy between Xbox and Steam like Sony did with Portal 2 on PS3.

I'm sorry, what?, Windows 10 also brings dx12 which makes significant performance increases on Windows based PC hardware, but only negligible increases for xbox one.
microsoft are bringing more and more previously xbox only games to pc, this announcement means the exact opposite, people on the fence are now more likely to ditch the idea of getting an xbox and buy a pc, and people who already have a pc have even less of a reason to buy a console.

Nobody is going to ditch steam and its now world renowned sales,  for a Microsoft attempt at taking steam over,  and nobody is going to switch even if xbox exclusive games are dangled as proverbial carrots,  either. 

No... I do not think so.
I believe it may be very appealing but there are way too many games on Steam for Windows 10 to take over.
It may become a better service at the end but Steam is not going to die anytime soon.

Dude, are you feeling alright?

Too late... around 100 million PC gamers already have Steam on their system and bought games on Steam (or Steam-codes at GreenManGaming, Humble-Store, indie bundles or key sellers). So why should they ditch their Steam-game-collection and migrate to the Windows/Xbox-Store? There is no Microsoft-advantage for this group.

Then there is the group of PC gamers who hate DRM and account binding or prefer retail games. If they missed out on Steam for that, they will probably also miss out on the Windows Store (DRM, account binding, no physical discs).

The only people Microsoft could win with the convenience of a pre-installed marketplace would be people new to PC gaming, but even most of them would be aware of Steam and its deals if they aren't cut off from the outside world.

Not a chance. And nobody should want it to happen after the scourge that was GFWL

Short answer: No!
Long answer: Noooooooooooooooooooooo!!!!!!!!!!!!!!!!!!!!!!!!!
Varied answer: Oh hell no!!!!!!!!!!!!!!!!!!!

1. Steam will get even stronger.
2. Microsoft tried the same with GFWL.
3. Valve went to Microsoft and many other companies to create steam. None helped. So valve took the risk by themselves.