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Nothing Phone builds a blue bubble iMessage bridge while Google and Apple fight over RCS
(www.androidcentral.com)
This is a most excellent place for technology news and articles.
They don't need your message contents to harvest data. They can see who you talk to, when you talk to them and the type of messages (text/images/videos).
Connecting people together can expand the amount of advertising impressions they can sell. If your speaking to someone at the same time they are googleing a new trampoline, Google can infer your likely to be interested in trampolines as well.
That's such fuzzy data that it really doesn't matter at all. Point is, my messages are encrypted, and Google can think I want a trampoline if they want to think that.
Plus you said Google wants everyones messaging data, but they literally aren't getting it. So no, that's not why they are pushing it hard.
Apple can also run their own RCS servers (my carrier does). They simply aren't supporting RCS because they LIKE that Android is shamed for having green bubbles. It would hurt Apple if their phones were more compatible with Android.