I think people with PCs and Steam will manage just fine.
Yes which is the ecosystem.
How is Steam their ecosystem? They pay Valve a 30% cut and those sales actively keep people away from Game Pass.
they are contributing to keep PC gaming firmly on windows 10
"Xbox" ecosystem extends anywhere where MS / Xbox can reach customers as part of their own ecosystem (part of why MS measures success by monthly active users in XBL). Steam effectively does that, as MS plop XBL right into the games on Steam - a platform that is effectively agnostic to various ecosystems.
Steam having a colossal audience that isn't engaging with UWP/Win10 store, and happy to stick on Steam, is something MS have demonstrably not been able to challenge, because PC gamers tend to enjoy choice, and the market there is entirely different - highly competitive platforms already exist with established audiences competing openly, with users happy to play in multiple places they prefer, or perfectly happy to ignore what they don't like since there is so much competition.
MS smartly recognise that on PC, they should support all the platforms that are ecosystem agnostic with large userbase, meet them where they are to engage with them. Long term once folks are connected to the Xbox ecosystem this way, they can start with the main goal - serving recurring revenue services such as xcloud and gamepass to this audience too, and whatever else. Long term, by meeting these customers where they are, they have better prospects for reaching this unique group of users and actually retaining them. Right now, its clear a lot of Steam users stick there because they enjoy its services. Clean integration with that is the best way to engage with these users when its clear Steam isn't dying any time soon. EA have done the same thing and others are following. MS will no doubt also do the same in a way that makes sense for EGS users, for example.
"30%" cut (or 20% for the games after they hit usual revenue mark, or 0% for Steam keys with whatever cut they arrange with third parties), is a really trivial cost to pay for the ability to establish Steam users into the XBL services
PC isn't the be all end all either. Xcloud and Gamepass will be on android, iphone, smart TVs etc soon enough, and will also become users that would be part of the Xbox ecosystem - paying customers.
This isn't happening in anywhere near the same way on Playstation since it much more directly competes with MS for console specific users. For those users no doubt MS will have a minority of ports / maybe more late ports, but primarily will push for those users to grab Xcloud / Gamepass if they not interested in another console. It's all about grabbing users wherever they are, and making more money.
There was a threat that has now died down that Valve (and Stadia) would push gaming on Linux via Vulkan and OpenGL.
Really unlikely that this is a specific factor, as Valve will/are continuing to support open solutions because that is best for its business. Their success is built upon the open PC platform, so its in their interest to make sure (at least as back up) open solutions see heavy investment while they are able to do so.
Meanwhile, it is true that Valve contribute significantly to the success of Windows. MS and Valve are still partnering forward looking things including VR. No doubt they'll align on a bunch of different things since Valve's focus is primarily on PC gaming.
