That said, VirnetX has also sued Apple about iMessage, so it's possible there's something to what you've said (but I've never heard it said before that Apple was going to open up iMessage).
Whether the culture has carried over, who knows.
Even as an iPhone user, I never would have thought iMessage was even in the top 5 or 10 reasons to keep using the platform. But I'm old, don't text all that much, and have unlimited SMS anyway.
I imagine all the cool kids these days are using alternative data-based services anyway. Snapchat, the like. For us old folks, WhatsApp, GroupMe, etc.
Yes and no, because friend groups are fragmented across services and iMessage or text is the common denominator, especially for group messages.
Apple doesn't make money through advertising they make it by selling phones. How does opening iMessages up to Android help them to sell more phones ?