IRC. IRC has been around for the rise and fall of everything else on your list, and a dozen more that we've already forgotten. I have used IRC for my entire life and I see no reason to switch.
As much as I love IRC, it is fundamentally incompatible with the way people use
IM these days. Even if you gloss over the lack of "modern" features, you're
still going to have a very hard time with vanilla IRC on mobile where keeping a
persistent background socket open is frowned upon (if not outright illegal).
Don't get me wrong - I love IRC, and have used it as my primary social network
for years. And after spending a full decade trying to switch people over to open
protocols, I've found the hard way that the network effect trumps all.
I just don't see the average Whatsapp user ever switching to IRC.
