Short-run profits are maximized by producing at the point where marginal benefit = marginal costs (I think you state as much: "If unit sold goes up by more than the lost per unit profit, then that's a winning proposition."). Apple might want to produce these if the cost equation works (it might not--see below.).
You're also addressing the halo effect ("If people convert to the Apple brand, then Apple is going to make tons more than just that first sale.") This is anecdotally true: after I bought my first iPod in 2004, I purchased my first Mac in 2005 and now purchase Apple products as I'm happier in their ecosystem.
With that being said, what will Apple cut back on to generate large margins for a $800 MBA? Do they decrease storage? Produce a smaller (9.6") laptop? I really don't see how they go to $800 and preserve margins.