The desktop form factor is declining significantly in popularity in the mainstream market in favor of laptops and mobile devices. The reason is obvious: laptops and mobiles are portable, and the average user (and even many pro users) do not need that much power in their local device.
It's still very popular in the gamer and professional workstation market, and likely will be for a long time. It's just hard to cram the processing power that gamers, hard-core developers, CAD and simulation users, etc. need into something as small as a laptop without creating what's comically referred to as a "ball burner" or "weenie roaster." I'm sure there's a female equivalent expression but it's even less polite. :P
Intel has many-core options for these machines, but they tend to lag a little behind. If you have deeper pockets and really want power you can always put a high-end GPU in a server board with 2-4 Xeons and put it in a tower case. Now you have a data center node with a 4K monitor on it.
Gaming is a niche market mostly for younger ages (people with "game rigs" drops rapidly after a certain age) and most hard-core developers use laptops -- the laptops you see in each and every dev conference are their main boxes, with or without external monitors.
As for CAD and simulation that's too niche to even mention. Of course some people will always have a need for a more powerful form factor. But if CAD users are part of the counter-argument for "it's not dying", then it very much is.
http://www.supermicro.com/products/motherboard/Xeon/C216/X9S...
http://www.supermicro.com/products/motherboard/Xeon/C220/X10...
They didn't make such a class of boards in the '8' generation.
Not dead yet!
The reason why typical GPUS have high power consumption is because they are generally designed for the desktop market where they can.
But you can have low-power discrete GPUs also.
For that matter, there is no reason why you cannot have a low-power discrete GPU, and a high-power discrete GPU.
And again, it's a whole lot easier to dissipate heat being produced in multiple places than all in one hotspot.
Where it matters is mobile. Even if you have a dedicated graphics processor, it makes sense to have integrated and then to have it auto-switch as needed. This saves on electrical power, reduces heat, and may extend the life of your mobile devices (since heat is damaging).
Ultimately if your want to force-on dedicated 24/7 then normally you CAN, but integrated simply gives you more options if you wish to extend the battery life of your device significantly.