That's cute. You really think that Steam isn't a big deal to MS? Bless your heart.
Microsoft has long wanted to completely control the PC Gaming ecosystem. The biggest leg up they had on PC Gaming was Windows being the dominant platform. SteamOS and the rise of Linux gaming is slowly starting to threaten that. There are plenty of people who say that has no chance of happening, but they clearly have never looked at history. No company is to big to fail. No monopoly is too big to fall. If they lose gaming, they lose a fuck ton of Windows users, which makes it harder to sell Office 365 subscriptions. If Windows marketshare erodes, that affects EVERYTHING at Microsoft. CoPilot, literally fucking everything. We are a ways away from that happening, but Linux gaming continues to improve. Microsoft would love nothing more than to get rid of Steam. So yes, Valve is a drastically bigger threat to Microsoft than Sony. Microsoft is acknowledging this by finally acknowledging that consumers see Windows as crap. For all of Microsofts flaws...and holy fuck are there many....they do seem to understand the threat of SteamOS.
As for why they sell their games on Steam. they don't have a choice. releasing their games exclusively on MS Store is a death sentence.
As for Steam being on the next Xbox. Yes, they have said that and I do believe that, but what will actually look like in practice? Microsoft is not going to make it easy to use Steam as they will want to heavily incentivize gamers to buy from their store. Don't kid yourself. I can see the OS going out of their way tohide Steam and make the user have to jump through some significant hoops to do so. Like having to specifically boot into a developer mode. I doubt it will be an easy experience.
As for Sony. Sony is more than happy to take their 30% cut of Xbox software sales and Microsoft is more than happy to get 70% of the cut from their games. Sony certainly threatens the existence of an xbox console, but it has virtually no threat for the Windows ecosystem. Valve does.
Did I paint the picture for you?