It's a little different in the console space. Didn't MS have 50m+ Gold subscribers during the peak of the 360 era?
Also, everyone and their brother has a streaming video service going, with gaming a GP like service can really only be pulled off by Nintendo, Sony, or MS. A lot less competition. We'll see how it goes.
I have no idea why GP can't go to 10 to 20 millions, but MS needs:
A: to sell at least double the consoles, good luck.
B: bring the value of the service far higher, good luck also.
Or a combination of the two.
Despite this, GP didn't boost Xbox sells nor boosted software revenues from it, in fact they continued to drop(as far as I know), which should be indicative of how much losses and income the GP can grant, even if it may not be reliable.
Problem is, long term effects are unknown. GP is sustainable because MS needs to, if somehow MS crush Sony like with PS3 I don't expect them to further go suicide mode.
Also, they would start again with user base due to SeX, and that's probably why they want to keep One as long as possible, because with that they keep GP subscriptions.
If my theory is correct, GP already "forced" Microsoft to limit its native next gen output.
But in general I hope it doesn't become a standard service for videogames, because as long as the lesser party use it that's fine, what if Sony and Nintendo got their versions with the same approach? Do you see all that third and first parties for 5 dollars\month for all platforms? I don't find it realistic.
I think is difficult to talk about GP because I see it as a very unstable service, similar to a bubble ready to explode if ever it gets big enough. I suppose it needs just a very narrow sweet spot to work without messing things up.