The laws of physics still apply. More powerful hardware is going to draw more power and produce more heat, requiring larger enclosures, active cooling etc. Look at the Snapdragon 810. It's going to get more and more difficult to keep increasing the performance of SoC without running into thermal issues and throttling. It's the same thermal brick wall Intel and AMD hit at the height of the GHz race.
More than anything, the rise of powerful mini-ITX PCs is the result of more advanced cooling technology. We can now stick 100W CPUs and 200W GPUs into a tiny enclosure without it melting through the desk. Unless you live in a shoebox, I see no reason *not* to go with full ATX. You get more expansion slots, more room for storage and other components, better cooling and importantly, they're easier to work with. I've never found myself wishing my Define XL R2 was smaller...
"Cloud computing" (using someone else's computer over the Internet) for compute-intensive tasks isn't going to work for the masses. For every 4790K+GTX980 in the "cloud", that level of performance has to be allocated physically somewhere. That physical hardware costs money to buy, produces heat and draws power. It works for simple tasks where many users can share otherwise unused CPU cycles, but not for more demanding ones unless you're willing to spend a lot of money for someone else to run that kind of hardware for you.