This "complexity hole" is a problem they've encountered before with the Pentium 4 line of chips. They ended up being too complicated and too costly to produce,

This "complexity hole" is a problem they've encountered before with the Pentium 4 line of chips. They ended up being too complicated and too costly to produce, so they had to scrap it and go back to the Pentium III design and iterate from there. I wonder if they'll ever have to back-track even further and use an ARM core but add on an x86-64 mode via microcode.