For me, it's just security, no other reason.
> There are plenty of other cross platform applications for messaging that fit the quoted needs.
Absolutely, and I encourage people to use them. Unfortunately, I can't force people not to use iMessage.
It's not about bullying (I've no doubt that it happens, but it's never happened to me). It's not about social pressure, I couldn't care less if someone wants to make a big deal over me having a green bubble -- don't let the door hit you on your way out. What it is about is the fact that I can't change my family members' behaviors, and the consequence of their behaviors is that all of their messages to me get sent unencrypted.
I would like those messages to be encrypted. I can't force them to use a better messenger, so it would be nice if I could on my end make a change that seamlessly, with zero friction on their part, causes their messages to suddenly be encrypted. No, I'm not buying an iPhone, heck off with that garbage. But I would be willing to install a separate app if it meant that my family members on iPhones could instantaneously have their messages encrypted.
Barring that, I can keep subtly encouraging them to use any of the other much more secure messaging services available, but... I mean, I don't control their phones. They are adults and they make their own decisions. And Apple doesn't really help here by marketing the Messages app as if it's secure while leaving out the fact in its marketing that a huge portion of the messages it sends have zero security at all. I tell people that we should swap to something else, their response is, "I don't need to, iMessages is secure." It would be secure if you were using it. But when you message me, you're not using it, you're using SMS.
> If end users care about “upgraded chats”, they can simply use one of those and ask those whom they message to also use those apps.
Like everything else in security, this boils down to the fact that people are apathetic and the people who are security conscious have to try and bend to meet them halfway. Beeper would have been a way for me to bend and meet some of the iPhone users in my life halfway. I'm not buying an iPhone, I'm not giving my family members an ultimatum that I'm going to stop responding to their texts if they don't use the messenger that I want them to use; that would be wildly antisocial behavior for me to engage in. So they'll send all their texts to me in plaintext.
As anyone who's tried to use Signal can attest, there is nothing simple about asking people you message to use a different app. And security in specific is a really hard sell for getting people to switch.
This is what I keep hammering when I talk about this -- Apple's position on iMessage makes iPhone users less secure. For anyone in my life who is security conscious, we couldn't care less about iMessage, we use actually secure cross-platform messaging services that allow us to actually encrypt 100% of what we send to each other. Emoji reactions do not matter, the problem is that iMessage can't send cross-platform encrypted chats, and Apple's position is that it cares more about whatever weird platform-exclusivity lock-in it thinks its getting than it cares about making sure the messages that iPhone users send are actually encrypted.
The motivation here isn't complicated, I want the iPhone users in my life to actually be secure rather than pretending that they're secure.
I'll note that the same problem also exists for Android. I'm not singling Apple out here, in practice Android users also send all of their messages to me in plain text regardless of whatever proprietary garbage Google is trying to pass off as message security nowadays. The same problem exists there, I can't get them off of the default messaging app. But on Android, there's not the potential of an app I could install that with no changes to their OS or setup would cause their messages to suddenly start being encrypted.