Skype is popular because it just worked, even through funky firewalls. The replacement would need to be better than Skype to gain traction with non-technical users.
Skype's overall quality has been on a very steady decline recently. From call quality, to call drops, to offline contacts showing as online and vice versa, to privacy concerns - Skype's position has never been weaker. It still got an obvious momentum, but it is actively pissing of a lot of its users.
How did the internet get so dumbed down? Cloud this and web app that and now nobody knows how to research or install any normal software. Or do anything that isn't shiny packaged at $10+ a month?
It's a bit smarter than just using an http tunnel.
Skype is capable of direct client-to-client connections, despite intervening NAT. It's pretty clever -- with the server's help as coordinator, the clients both initiate the connection, causing their own NAT routers to accept the inbound packets from the other side.
I'm not keen on some of the recent changes to Skype but I don't really consider "Skype for Linux" to actually be Skype. Skype is actually the only reason beyond dev testing that I keep MS Windows.
Skype for Linux is the only software I've installed in at least 8 years, AFAIR, that has crashed my desktop session.
I wish all it did would be to crash my session. Mine would go into an endless loop or something, consuming 100% CPU. Everything would still appear to be working fine, only I couldn't make any calls, my chats wouldn't be delivered, etc. This happens around once every five minutes, and I have missed important meetings because Skype had hung and I was wondering why the person I was waiting for hadn't logged in yet, only to be asked where I was later.
It's the worst sort of bug, because it leads you to believe it's working fine, when it isn't. Skype for Linux is the reason I don't use Skype any more.