...aside from sales blips where the fanboys (I count myself in the group) wait for the latest updates, what is the cost/benefit to Apple of constant updates, factoring more platforms to support, engineering costs and marketing costs to change?
There isn't any. I'm not saying there would be. Apple, however, is a company that claims to be ahead of the pack. To live up to their high standards, the least they should do is keep their units modern. 6 months is roughly a quarter of a Moore's Law cycle which would imply performance increase of at least 25%. This should be the minimum update cycle for any technology company, especially one who claims to be so cutting edge. Apple shouldn't be able to escape such a claim just because 90% of their user-base is ignorant.
Besides, if there weren't fanboys and prosumers keeping companies in check, how would anyone be able to trust that they're actually getting a better product?