The 8051 family also has a huge amount of support tooling for it -- compilers, validation tooling, etc. For a lot of jobs where the 8051 lives, a new architecture would just mean an expensive set of validation tests for no real benefit.
Wow, you're right. I figured an 8051 was popular, but its derivatives make up >50% of the embedded market it seems. I am guessing most clones offer more than 128 bytes of memory.
Yup. The serial relay boards I resell are controlled by an 8051 variant. Those things are everywhere