Do you have any real market figures about how many mac users prefer modular desktops? We know however that Apple sells about 5 millions Macs per quarter despite not being modular. The agenda of Apple is not different from many if not all vendors inside and outside the IT field. I think it is very few products (think also outside the IT sector) that are upgradable and user servicable nowadays. For good or bad that is the way the market is going and Apple is tagging along.
Mostly anecdotal, many mac users I know personally would rather get a desktop, but since Apple severely neglects them not updating them for 3-5 years, they compromise and get a laptop or imac. I've seen also an increasing amount of mac users online that are choosing iPads as their mobile choice and would rather have a desktop mac instead of a macbook to complement it.
There is also the fact that the Mac Pro and Mac mini despite being obscenely neglected, they still keep a ridiculous great resale value, what does that tell you?