> They literally can't understand what iMessage is literally just another app, with the only difference is what it's default.
...since all the answers focused on it being a convenient progressive enhancement to the SMS experience, I'm pretty sure they do understand this.
> I would point out to "I do not need to ask them to download/register on iMessages".
Yeah, because the selling point is the aforementioned convenient progressive enhancement. You try to text someone, and if you both have iMessage it automatically becomes better. Would it be nice if Apple had some sort of framework so their messaging app let other messaging services integrate into it so that it wasn't only iMessage that benefited from this quiet upgrade? Yes, but I can see how that'd be an excessively complicated app to maintain.
> There was quite a shock for some people what Apple actually gives up info on their customers when the law enforcement comes with an order.
You're a bit of a goalpost-mover, aren't you?
To engage on the law enforcement and privacy aspect despite it being unrelated to the prior point: falling back from one logged and requestable messaging protocol (SMS) to a different slightly less-logged and less-requestable method (iMessage) is fine. Plus, unless Apple has been actually lying in their marketing, iMessage is end-to-end encrypted, with the only backdoor being that law enforcement can get a search warrant for your unencrypted backups. But there's also a way to turn on a fully encrypted backup, once you go through sufficient warnings that customer service won't be able to help you if you forget your password, and if you do that it's legitimately secure from law enforcement.
> [0] showerthought: there is an intersecting Venn diagram of people who doesn't use Signal 'because phone number' yet use iMessage.
It's because people can have different goals for different products. Signal markets itself as a secure messaging service, with a notable focus in activist communities, and so having to tie you to a super identifiable piece of information is a bit strange. iMessage is just texting-but-better, and automatically steps in when the alternative would have been sending a SMS.