The problem is that there's not enough money in desktops. And even less in cheap desktops.
This! This is Apple's problem -- the amazing profit in creating a (temporary) monopoly by introducing a product that creates (or revolutionizes) an entire market. The iPod started the process, by taking mp3 players and making them mainstream products for ordinary consumers. The iPhone continued this, by recreating the concept of a smartphone in such an appealing manner that it single-handedly upended the entire market.
When you can do this, you can make a lot of money. When you can do this multiple times, you can make a mountain of money. The problem, of course, is that you can't
depend on being able to do this consistently.
Apple has designed their entire business model around always finding the next breakthrough product. It isn't a question of adding new devices to their existing product lines; they now actually
depend on finding a new iPod or iPhone every few years. If they tried to survive on Mac sales alone, the company as currently structured would collapse.
This is Apple's true failure: they have become addicted to enormously profitable products. Profits that, by their nature, only exist for a short period of time. The iPod is basically gone now; the aTV and the watch seem to have already plateaued; and the iPhone may be reaching a peak as well. But the Mac, a device that has been a more-or-less reliable seller for three decades now, is useless for today's Apple, because the profits it generates cannot sustain their current business model.
Desktop computers have been a facet of daily life for decades now. The truth is, they will continue to be a facet for decades more; it's just easier to get work done by sitting down at a desk and concentrating, than by flitting around with the latest gadget in your hand. Apple may have lost sight of this truth, but other companies will continue to produce PCs, and people will continue to buy them. There
is still money to be made here.