The question to be had here is if exclusives are truly a viable strategy to gaining market share in the console market, and there's a second question to if a dominant position in the console market is at all needed in the current gaming industry. I think the answer to both of these is no and based on their actions I know that Microsoft thinks that regardless of what either of us say.
I think that's generally correct on what matters. However, I would cast that differently: what matters is not purely a function of what the market wants. That's VERY important for sure, but strategic decisions must also factor a company's strengths and weaknesses relative to the competition, and then set a strategy that leverages how those strengths and weaknesses interact with what the market wants. The right strategy for Sony is not the same as the right strategy for MS, because they have different strengths and weaknesses.
I think what you're really arguing (not trying to put words in your mouth, but I think this is an accurate rephrasing, please correct me if you disagree): Microsoft makes most of its gaming money from software sales and is seeing growing sales from GamePass subscriptions, which sells more on Windows than Xbox. Therefore, locking exclusives just to help sell Xbox consoles is a wasted opportunity to sell to all the PS owners, which wouldn't really adversely affect their core sales and business model around gaming and subscriptions anyway. In fact, selling to PS exposes more users to their games and with gamers moving toward Windows/PC/other gaming, better to have that exposure than avoid PS just to help market-failing console as a shrinking userbase in a static-at-best overall console market.
I understand and appreciate that. However, what I believe that's missing is:
1. Microsoft now has a strong position with in-house studios that it never had before.
2. There is a market value (hard for me to quantify) in owning the family room/living room audience. This is a bit of an intangible, but think of it as the default communication system with hundreds of millions, perhaps billions of users. As long as people have TVs in their living room and spend notable time on that every week, owning the chief way they get content on that device is an incredibly powerful asset.
3. If Microsoft can leverage #1 to strengthen #2, this a good thing to do. It's smart strategy. This doesn't need to mean 100% of MS games are exclusive, just leverage it in a smart way to gain console market share. This is why they do their "Everything is an Xbox" campaign, which makes some sense, more than also releasing for PS.
To tie this into the Steam OS discussion, where it looks like you and I are on the same page: even if it grows profits in the short-run to cross release games on PS, sacrificing controlling access to the customers is how a company loses relevance in the long-run. Market dominance has significant advantages beyond the short-term profits. It lets the company in that position set the standards, which can in turn play to its other existing strengths (think Google's search dominance and their ability to set both how advertisers and consumers use search). It gives them greater negotiating leverage with suppliers and partners (think about BG3 not running on Series S -- that's a consequence of low market share).
Microsoft definitely sees the writing on the wall with the most popular gaming storefront on PC building a successful console like operating system that uses Linux and has really good emulation for games. More so when SteamOS based devices are entering the market with fairly low prices. Like Valve is selling SteamOS licenses for cheaper than Windows and as a result we're getting cheaper PC gaming handles that don't run windows (like the legion Go S). They'll have to do more than ever to compete in gaming with both Xbox and Windows.
I think their failure to respond to Steam OS is due to the same mindset as their releasing what could be exclusives on PS -- they don't appreciate the value of controlling a market. For a company with MS experience, I find this bizarre. My best guess is it is a consequence of the prior problems they faced 20-30 years ago when the government successfully attacked MS for monopolistic behavior. But these decisions by MS go way past that to just being bad, destructive strategy.