Look at the evolution of the Pentia:
4004 > 8008 > 8080 > 8086 > 80186 > '286 > '386 > '486
> Pentium > PPro > PMMX > PII > PIII > PIV (etc.)
Intel did try to make a few other mainstream CPUs along the way. In the 1980's, the i432 - Flop. In the 1990's, the i860 - Thud. And now the Itanium (nicknamed the "Itanic") looks like another silicon smoking crater. Design partner HP recently announced they're pulling out. In order to get a marketable 64 bit design, Intel had to copy, yes, copy, AMD's x86-64 design. And it's yet another remote descendant of the 4004.
This isn't to say the art of processor architecture is dead. IBM/Apple's Power, ARM, MIPS, etc. all seem to be doing well in their various markets. It's just Intel seems like it will forever bound to that first design.