Why would you care as a consumer?
Only trolls are interested in sales numbers, even if Apple sold a billion of them, it still won't be enough.
Why wouldn't I care about sale figures? For a product that is driven by an ecosystem, I would want to know if there was enough sales to warrant third party developer interest. If sale figures / marketshare was low, then 3rd party support would probably slow down because there aren't enough users.
Additionally, how long can a product keep being sold for if sales are lacklustre before it is canned?
I'm not spending $500 on a product that is performing badly in the market place.
If Apple sold "a billion Apple Watches", I'd be very happy.