"as long as both parties have whatsapp". Exactly my point. And if both parties don't have it?
"imessage is not cross platform because you cannot run it on a windows, linux or android". I guess I better tell my Android friends to stop messaging me if that's the case LOL
That's not iMessage, that's SMS. Not the same, even if it appears in the same app on Apple devices.
SMS might be free in the US, in many contries around the world, it is not free and can cost as much as $ 0.10 per message. That's where apps like whatsapp come in.