They sure exist! But Google's search has like 4/5 of the market.

If you optimize for the bottom line (as a business should), what will you pay most attention to?

I think his point was that Bing implemented AMP voluntarily, without being pressured as you suggest publishers were: https://blogs.bing.com/search/September-2016/bing-app-joins-...

Bing's primary pressure is a need to compete performance wise with Google. The fact that the implementation is bad doesn't mean they don't need to match on the metrics.

To play devil’s advocate: the reason Bing can implement their own AMP cache and conform to AMP pages so easily is because AMP is an open project.

Nobody said the problem was AMP's code being available. The issue is the way Google is forcing their framework's dominance using their search monopoly.

Does Bing host the pages on their own servers? Hosting your content on their servers could be a motivation for joining. If Google is going to appify your websites on their own servers, then Microsoft doesn't want to be left out. They can both put giant back buttons on your website that take you back to the search engine results instead of deeper into the website.