What's stopping Bing (and others) from using AMP and Microsoft (and others) from building their own AMP caches?
The main thing about AMP is that it gives consent to the search engines which is why Microsoft is happy to use it. The consent allows search engines to display content without directing users to the publisher platform. And therein lies the rub.
AFAICT [0], the AMP project is an open source project directed by its Tech Lead and Core Committers, which are a self-sustaining group (with Google, Inc. appointing a new Tech Lead in the event there are no Core Committers), even if the initial set was set by Google, and there is no barrier to Microsoft participating as fully as anyone else in that project.
[0] largely, from the project's own governance document on github, https://github.com/ampproject/amphtml/blob/master/GOVERNANCE...
When FB moved to instant articles, Google moved to amp. It was the opening they had been waiting for, for a long time. AMP was an opportunistic play--a damn good one for Google.
It sucks for publishers but they are forced to play along because of you are always at the mercy of your dumbest competitor.
Users may win but this isn't something publishers want or desire. Building faster pages is easier now than ever. Not as fast as AMP or Facebook Instant but still.
Over time this will lead to commodification because of lack of standards. Bing and others will further lose their lead to AMP. Users will like the new UX. Publishers are confused.