It’s not that people need a blue bubble to indicate that the other party has an iPhone. That would be silly. It’s about the features afforded to you if you use iMessage, which were revolutionary for a phone when it was introduced. remember that this was introduced a couple years after the iPhone - it wasn’t a first gen feature.
Knowing if my recipient received the message, that my recipient will receive the message at all, and seeing if they are replying — these are the reasons I’m an iMessage fan. And perhaps most importantly, on my Mac I can respond to and send such messages as well. All of this works works with wifi connection only, no cell signal needed. Making it usable on airplanes etc.
None of this works with the dreaded green bubble. None of this.
With green bubble, I can’t guarantee my messages were sent in order. If I send a message with a photo it just as likely won’t be received at all of their phone can’t receive it in the proper time or if they are in a bad signal area. Suffering through this is not worth the effort to talk to most people (and call me whatever you want but it’s true and you know it).
It’s not about what device my other person has. It’s about being able to communicate with a technology that doesn’t resemble a dial up modem.