When all your friends are locked in on iMessage, you can't just not buy an iPhone. Apple is using lock-in so that you can't simply move to Android.
Honestly, I don't even know how that would be an argument for locking you into an ecosystem. It's not like text messages are an ancient technology that modern iPhones will not support.
https://www.androidauthority.com/green-bubble-phenomenon-102...
I know multiple people who refuse to communicate unless it's on an app like Signal or Telegram, but I don't know anyone over the age of 17 who cares that my bubbles are green instead of blue. And even if I did happen to know people like that, I still don't see "well some people will find literally any excuse to be exclusionary" as a good argument for iMessage locking users into their ecosystem.