Re: 8051: one of the most widely used
The 8051 architecture and the miriad of variation including the 8052 variations were used in things that needed just enough processing power and no more, especially for controlling simple display and button inputs. A couple of interrupts and a few bytes of memory can support a lot of functionality.
Things like corporate PBX handsets with a 2x16 display and 8/16 buttons almost certainly had an 805x controller, and less obvious things like petrol pumps had an 805x in them somewhere.
Even the Tini Java computer on a stick had an 8051 based core (https://en.wikipedia.org/wiki/Tiny_Internet_Interface)
Think it was a good enough architecture with enough scope for variations of features/expansions that it could be considered an industry standard, a developer and the tools developers just had to understand the variations to be able to use them effectively.