Earlier quoted context omitted.
They were if you're willing to overlook the requirement -- which WhatsApp shares -- for the other person to use the same app. (Well, protocol. Gaim handled AIM and MSN fine.) The only thing SMS offered was that the other person didn't need to be running anything. WhatsApp didn't offer that, and also didn't improve on the other messengers of the day.
You’re missing the fact that WhatsApp was actually available for essentially all phones (even in 3rd world markets) and that nothing else is needed. AIM and MSN required an email address which means you need one, as well as an email client on your phone, not something that all people want or have. The signup process wasn’t even always available within the device, you might need access to a desktop to complete signup.…
And it still is. Even cheap feature phones often have WhatsApp support.