I think you're looking at this from the perspective of why you don't want them to be multiplatform.
You think it would kill their console business, but how many Sony fans said the exact same thing about Sony games being ported to PC in the past year alone? A ton did just that, and they were all 100% wrong. On top of that, didn't you suggest they don't need access to 150-200 millions PS4, PS5, and Switch consoles, because they can just sell those people something like a firestick type of device instead? If it worked as you suggested, it would simply accomplish the exact same thing you are afraid of (devaluing Series hardware), at a greater cost (having to make them), and take years and years longer to reach the end goal (moving subs). Never mind what happens to hardware if streaming becomes the primary means of distribution.
As for the lost leverage in drawing in new users, again, look at PC. No one who uses GP on PC actually needs an Xbox. But MS still sells as many PC users a GP sub as possible, and there is no evidence the platform has suffered for it. On a similar note, I'm curious as to when Xbox started focusing on console sales again in the eyes of its fans? I'm talking generally here, and not about you specifically. I'm just having flashbacks as I type to a few million snarky posts ever since Bethesda got bought, that its all about GP these days. Every single day, every single enthusiast website's forums / comment sections, its all about GP, its about GP. Now, suddenly, MS has the leverage it needs to expand GP to the last two closed gardens, and GP takes a back seat to hardware?