It's not exclusively an iMessage problem, you're right but it is for teenagers. This is probably the weakest point of my position but teenagers in my experience don't peer pressure over tech specs but off obvious shit. iMessage directly shows you who is a green bubble or blue and who is going to send you compressed as hell videos and shit because of it.

Apple deliberately created a distinction of their own making with iMessage by making the bubbles different colors and refusing to support features to improve green bubbles for over a decade now. But the people who are razzing and peer pressuring aren't exactly going to understand that iMessage wasn't special tech or anything. RCS was out three years in advance to iMessage and all Apple had to do was allow it on iPhones.

Now outside of teenagers I have never really seen someone pressure people to have an iPhone. It's a really childish thing to do because they're really the same damn thing for the most part.