Those were not simple, seamless, cross-platform, drop-in replacements to SMS though, which is what OP was referring to by “text messaging.” Sure, clunky systems which had no chance of mass adoption existed before.

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. They both require you to remember and accurately share an extra piece of info you have to memorize (email or username). They were both extremely slow, clunky and unreliable compared to WhatsApp.

Essentially, 95% of the reasons WhatsApp was successful were things you glossed over here.

> You’re missing the fact that WhatsApp was actually available for essentially all phones

And it still is. Even cheap feature phones often have WhatsApp support.

I haven’t used the services you mentioned but maybe WhatsApp was easier to sign up for since you just needed a phone number?

That's not a replacement from SMS since those aren't based on phone numbers. If you have the phone number of somebody and they have WhatsApp installed you can message them immediately. If you both have AIM or MSN installed you have to exchange your handles through some other channel. That's a massive difference in terms of usability and friction to get started.