The phone network in the US was basically the same 50 years ago.[1] It took a major antitrust fight to bring about "cooperation". So strange, folk strenuously defending obviously anticompetitive conduct.
It would be proper if iMessage would be the only messaging service phone users can use and installation and usage of the others are restricted.
But anyway, my whole comment is about making commercial messenger with the expense of other product (aka. backend services of Apple) without permission, cooperation or anything else. There aren't official public APIs for iMessage other than for Business use.
[1] https://en.m.wikipedia.org/wiki/Breakup_of_the_Bell_System