One of the reasons the 8051 is going strong is that it is (more or less) open source. You can build a 8051/52 in a FPGA or ASIC, add new instructions and not pay a fee. Many silicon houses use 8051s for that reason.
SI Labs has 8051s that run 100 times faster than my first project. Some of the parts have more memory pointers, hardware multipliers, DSP instructions, and more. All the parts I have used recently have internal PROM, RAM, TIMERS, eepron, ADC, etc. I used one that came in a 2mm by 2mm package.
Some 8051 hide under other names.