Why would it take 2 more years to develop a desktop?
it shouldn't, and it doesn't.
The only time these sorts of computer projects take longer than a year is if they believe they can do something different and have to basically invent/innovate something completely new.
The Surface pro for example took almost two years (evidenced by the generation old PC hardware). Because it did several things new and different. That display for example.
SO, if a PC manufacturer wanted to use generic / Commodity PC hardware, they could probably have a tower and device out in 6 months, with most of that sourcing parts.
so if Apple is claiming 2 years from now. First, they're just starting. Second, they are still intending to deliver a "Pro" machine that is likely proprietary and attempts to push some boundary, preventing them from using off the shelf parts.
i'm not passing judgement. I'm torn either way. The "Pros" are going to be perfectly happy with a Pro tower using semi-off the shelf parts. But, I'm also curious to see what Apple envisions. While I don't always agree with their outcomes, I do appreciate that they try. The Mac Pro form wasn't a failure due to it's form. it was a failure because Pro's weren't looking for that. If it was the "Mac Gamer Edition", and priced ~$2,000 and had an i7 with 1060's in it SLI'd, I GUARANTEE YOU it would fly off the shelves.
Most people I know who rely on compute power for business frankly, don't give two flying pisses about the looks, many running full servers anyways. Compute power first. look and feel second.