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.