There are many downsides when you don't control the standard. Microsoft is not in the meetings when Google PMs decide the roadmap nor will they have any clout in directing its future or adding its own features. It's merely a consumer.
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.
> Microsoft is not in the meetings when Google PMs decide the roadmap
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.
What's stopping Bing (and others) from using AMP and Microsoft (and others) from building their own AMP caches?