THAT was what was needed for mobile devices. I remember trying to use MSN messenger when I was on WAP and it sucked. And then I remember Skype on iPhone sucked for messages too. SMS also wasn't great if you were remotely international and had friends and family not in the same country as you.
It took two whole years for Skype to rearchitect away from peer to peer (understandably so).
To this day, when you want guaranteed delivery, in order and without duplicates WhatsApp runs circles around its competitors.
Compare that with Slack which both on desktop and mobile regularly has weird issues with successfully delivering a message.
Ultimately avatars are nice to have and later on top of a reliable messaging service but otherwise I'd take boring old text over flaky rich messaging every day of the week.
idk about slack et.al. but telegram does that with native clients for linux, android, ios and windows. even media-sync is pretty much flawless ime.
And that's ignoring that niche it filled where people were identified by emails, and the particular social rules of knowing that if you went online, it meant you wanted to spend a while talking to people and were available to do so. Discord is kind of similar in that last aspect, but it's enormously complex for the average user and intended for servers, not for 1 on 1 conversations with your contacts. I've always wanted to build a clone of Messenger just because of how much I miss it.