[adgjlsfhk1]

8 and 16 bit are dying pretty rapidly. You can make some pretty tiny 32 bit CPUs (e.g. https://github.com/YosysHQ/picorv32 is a RV32 with only a couple thousand transistors). On a budget optimized process node (e.g. 28nm), the core is absolutely tiny and all of the cost comes from the (s)ram.

[SAI_Peregrinus]

Yeah, I expect them to get relegated to Chip–on–Board (the little black epoxy blobs you'll see in 4-function calculators, cheap multimeters, & the like) ASICs as integrated MCUs and as soft cores in some FPGAs. I'd only go with an 8-bit or 16-bit chip at the extreme end of cost–constrained ultra–high–volume design already, 32-bit MCUs are cheap enough for everything else.