They have to carry them before anyone can buy them. The carriers will be promoting BB10 as an alternative because there is a market for them and they feel it.
To use a simple but I think highly appropriate analogy, people will buy BB10 for the same reason some people drive Volvos and Subarus (some BMW drivers as well); they are dependable and mature products, you don't see them absolutely everywhere you turn and most of all, you don't see idiot teens driving them.
That's my take, but maybe the reverse happens and teens dump their iPhones for a BB10 device just cause its cool and fast and different, and then somebody else comes along with something even cooler, and so on. Revolutions have a way of going on much longer than most people think. In any event, to assume we are at the end of history with the smart phone or mobile computer revolution is to reject everything a revolution is about and what made Apple what it is, Steve Jobs himself turned that notion on its head. There is room for BB10, you'll see. It's like Bob Dylan post 60s, no more multi-million copy selling records, but extremely loyal fans. That said, I saw him a couple of months ago, horrible, absolutely horrible. Granted he's a geniius, but he holds his audience in contempt. I take back the Dylan analogy and stick with Volvo.