What are CPU designs which are not fetch/store but operate directly on RAM?
I only know about the design of the Nintendo Entertainment System (NES), where the CPU does not have registers (AFAIK) and operates directly on RAM, with fast access to low addresses in the RAM.
What CPUs/Systems do you know, which also do not do fetch/store for their operands? Which systems are out there? Why do CPUs like RISC/Arm/AMD64 use fetch/store, what are the tradeoffs? Are there different architectures for CPUs working on operands outside of fetch/store, DMA and stack machines?
Registers would only add very little amount of transistors to a CPU with huge gains, so even at the low end it isn’t worth it.
These days low end is also crazy, you can get full fledged 32-bit microcontrollers for pennies.