People have more or less converged on ARM for large things and 8051 for tiny things. I suspect the remainder are hiding in culturally isolated niches; I don't really know why people are still targeting PIC16, for example.
There's also the DSP subset where people need to write inner loops in highly platform-specific assembler. e.g. my employer Cirrus has our own "Coyote" architecture for which we have a C compiler: https://statics.cirrus.com/pubs/proDatasheet/CS4970x4_F1.pdf
"Cheap" isn't just about BOM price, the power consumption matters too.
There's also the DSP subset where people need to write inner loops in highly platform-specific assembler. e.g. my employer Cirrus has our own "Coyote" architecture for which we have a C compiler: https://statics.cirrus.com/pubs/proDatasheet/CS4970x4_F1.pdf
"Cheap" isn't just about BOM price, the power consumption matters too.