For anything designed the last 5 years it's more than likely that those pesky old 8051s have been replaced by ARMs, Coretex M0s, M3s and M4s
Actually, its not. For many applications, this would require rewriting of the software stack, for a chip with roughly the same die size and possibly less funcionality. 8051 is a microcontroller, not a microprocessor.
And that is exactly what those ARMs are, they are microcontrollers. It's several years since the ARM microcontrollers started to dip below the $1 pricetag becoming a valid cometitior in most microcontroller designs. Those cheap ARMs have more or less taken over the market for 16 bit micros, and are doing heavy inroadds in the typical 8bit martkets. If you have started a microcontroller based design the last 5-10 years and not included one or more ARM microcontrollers in the evaluation process, you have not done it right.