The core is simple, small, cheap or even free, requires few resources, has plenty of tool support, is well-understood and well-documented, and is easy to debug and deploy. The 8051 is perfectly sufficient for many simple embedded applications that only require an 8-bit micro.
It's the instruction set that has been retained, not the silicon design. The variants these days are more power-efficient and powerful in terms of MIPS and peripherals, and have indeed benefited from years of R&D.
Yes but it's a bitch to program, multiple memory hierarchies and address spaces (at least 3), only one index register (hard to move stuff), and enough variants that "8051" is more of a species definition than of a particular architecture
(disclaimer: I sell an 8051 based product, have sold them in the past - never again)
It's the instruction set that has been retained, not the silicon design. The variants these days are more power-efficient and powerful in terms of MIPS and peripherals, and have indeed benefited from years of R&D.