One really obvious thing for Apple to do, which has always been a really obvious thing for Apple to do, would be to distribute a version of iMessage on Android.
Given the lack of emphasis Apple puts on iMessage in their marketing, it seems unlikely it's the single most important selling point. On the opening page for the newest iPhone, it's mentioned in a single sentence in a paragraph on Security down near the bottom of the page, hardly pride of place.
https://www.apple.com/iphone-11-pro/
Granted, I don't see Apple opening iMessage for other platforms for a number of reasons, including product differentiation, but at least Apple seems to think there are many more features to highlight before getting around to iMessage.
They have no incentive to bring it up more, the green bubbles are burned into the psyche of every Apple user.