Form factor is just it. Arm powered devices are great; cell phones have reached the potential to really make a persons daily task even easier and on the go. I remember back in the day when the first 486 hit the market. 486sx! Running 25Mhz was insane. As my signature says, now my phone is faster than my computer only 8 years later. At that time I was running a single core Athlon XP 1700+ clocked to 2.1Ghz. May have a faster clock rating; but this small little battery operated processor in my phone could out run it in a number race all day. That's the beauty of dual core.
Now; as for what me or you may use our desktop/laptop computers for; the entire side of "going mobile" can only go so far. That data has to go somewhere. Massive servers that hold 100TB of information just can't be held in the palm of your hand (at least not yet). Software can only go so far; as can silicon dies in processors today. Multi-Core is just another way around a physical electronic element in processors. But as with everything; more cores = more space. Even though my i7 quad core processor is only the size of about a quarter; if we watch the next decade... My processor would be the size of a nickel while the newer 32-core (or whatever it is at that time) will be a bit larger.
If anything; I want to see that "crystal"/light powered cpu idea come to life. Speeds that just make anyone cry; and no heat? Still a bit far fetched; but so is powering my device a mile away from anything.
But the future is the future; and honestly I'm excited to watch the next 15 years of computing. No doubt my cell phone will be more powerful than this home server.

Just hope that thing has 16GB of ram too!