Far from addressing all of the issues raised here, a possible solution to one of them -- lost sales through Xbox store of console exclusives -- could be addressed by MS simply contractually requiring of Valve and others that MS games can not be purchased for Xbox play through Steam, etc. That would be a small concession by the store owners to gain access to Xbox players, so seems an easy deal to strike.
Jez, you dismissed MS making these stores only available behind a GamePass paywall, because currently Steam and others do not require payment to access their games, but I don't think that's a reason not to do it. To your point that the changing model could be the greatest danger, the simplest solution that protects MS' overall profitability without breaking anything that they currently do is precisely to make access to these services on Xbox one more benefit to subscribing to some flavor of GamePass.
I would love to subscribe to GamePass, but currently do not, just because they don't offer a family plan yet and each of my kids has his and her own Xbox, so I can't add the service for one of us without buying a GamePass subscription for each kid, which I'm not willing to do (way too expensive). As a result, we're all blocked from multiplayer (except Minecraft via LAN). And as someone w/o GamePass, I can easily say I would not be upset with MS if they added Steam access AS A NEW FEATURE and made it only availbale to GamePass subscribers. I know I've made a choice not to subscribe, so my expectations are that I only get single-player versions of games through the Microsoft Store. If I want anything beyond that, I need to step up to a GamePass subscription. I suspect that's a fairly common perspective on this subject.
Note that this would also remove the need to do anything about the problem in the first paragraph above: as GamePass subscribers, they'd already have access to the game directly from MS anyway because all first-party MS games release on GamePass on day 1.