Really the only thing that was keeping it alive in my opinion was the nostalgia factor. Makes much more sense to make everyone use Skype, I don't think they even had cross platform chat which probably could have helped ease the transition.
But they did have a protocol supported by many clients. This is very difficult/impossible with Skype. Though at least it works on the desktop, unlike whatsapp, the most idiotically crippled IM client of all time.
And yet works on cheap flip phones across the world
Sadly Pidgin (https://www.pidgin.im) the library used by most alternative chat programs doesn't support Skype.