One of the major differences is that back then you could have complete single-cycle random access to any byte of memory, and the 6502 relied on that. Memory is

One of the major differences is that back then you could have complete single-cycle random access to any byte of memory, and the 6502 relied on that. Memory is a complex ball of wax today, with wide bursty buses and caches. While a 1+GHz 6502 could certainly be built today, it would have to be paired with on-chip SRAM for its main memory, which would dominate the CPU in transistor count, power draw, and silicon area. The transistor count itself was very low even for its day, so while that isn't my forte, I would guess that gains in that metric wouldn't be all that great.