For example, when RSVPing to a kid's birthday party, other parents' numbers are inevitably blue. When selling and buying items, the contacts for those sales have always been blue numbers, it's rare to encounter a number that doesn't "turn blue" when I enter it into the "to" field
I would say maybe 5% of the people I know and text use Android. For one of those people I use Signal, one other has asked me to use Facebook Messenger, one has asked me to use WhatsApp, and the remaining few use SMS. It's a pain to use three separate apps to message just these three people!
One of my cousins switched to an Android phone. This broke our long-standing group message in iMessage, so she was no longer able to be included in it. After two years of this her siblings simply ordered her a new iPhone and she is back in the group chat
Getting everyone to move their default messaging behaviour for one person is a huge ask. It was easier for one person to just relay the group chat info instead, but when this became annoying, it was even easier to buy her a new phone