8051 was once big in industrial applications like PLCs.
Somehow I doubt much has changed. In such applications, reliability + maturity of hw/sw ecosystem is much more important than raw speed or design innovations a competing architecture might bring to the table.
So 8051 based parts may see the occasional process shrink, addition of new peripherals, or new IC packages, I/O pin counts, operating voltage etc.
But I'd doubt any designer worth their salt would dare touch that core architecture unless their life depended on doing so. :-)