So let's make this interesting, assuming a ground up rewrite of an entire highly optimized web application stack - from the metal on up, how many normal boxes full of server hardware could really just be handled by one? 2? a dozen?
I'd be willing to bet that a modern machine with well written, on the metal software could outperform a regular rack full of the same machines running all the nonsense we run on today.
Magnified over the entire industry, how much power and space are being wasted? What's the dollar amount on that?
What's the developer difference to accomplish this? 30% time?
What costs more? All the costs of potentially millions of wasted machines, power and cooling or millions of man hours writing better code?