Consumers probably have "four or five devices" that do WhatsApp already
Not officially. WhatsApp officially works only on one's primary phone, the account being tied to its phone number. You can synchronise a web app view from a desktop/laptop, but they've made it intentionally awkward to do the same from a tablet. There are of course workarounds for all this, involving number fakers and whatnot, but not for the faint-hearted.