Those aren't mutually exclusive. Some of the sophgo and bouffalo chips have 8051s for always on cores, and riscv for the main cores.

They didn't choose 8051 there for experience, but because it was a tiny core with a decent IPC they could license for a small part, then focus on the main cores. I wouldn't be surprised if they eventually switch to riscv there too.

Also, these 8051 cores tend to be extremely diverse. I don't think I've come across cores from different manufacturers that were actually compatible for real code. They all seem to want to handle accessing 16/32 bit memory differently, have different interrupt details, etc.