Whatsapp's success is more down to the fact it run on everything rather than just phones that are realistically only something wealthy nations can buy.

If you think about how many people are in the world then look at the marketshare of the iPhone or even Android. Neither brand comes close to being something that everyone uses. If you want to target everyone you need to spread your support. That's what whatsapp has done. The phone number makes it a bit easier (excluding the fact it means whatsapp is not really usable on computers or tablets) but if every phone came with whatsapp and asked you add an email address or pick a user name I doubt that would have made much of a difference. It's the fact you can be poor person in a third world country and get it on your phone more than anything else.

Arguably that's the problem with a lot of tech including websites. They believe everyone has a similar computer or phone to what they have in their nice comfy silicon valley office. So they fight for the marketshare amongst their wealthier people in the world. WhatsApp just went for a different approach and targeted many more people.

Anyone with an iPhone can be contacted by their phone number anyway unless they've explicitly told it not to use their phone number.

As it stands anyone can contact me on iMessage with my phone number or one of 4 email addresses. So someone doesn't have to remember my phone number. They just have remember one of 4 of my methods of contact which seems easier than saying you must remember their phone number.

And for iMessage to receive the same level of support as Whatsapp it basically has to target all phones and why would apple do that? They're not a software company. They're a hardware company so basically saying "You never need to buy an iPhone because the thing you want is available on everything including dirt cheap nokia phones" seems counter productive.