I would think a lot of manufacturers are looking to RISC-V for that kind of eventuality. But in the meantime, and looking at that chart, it seems a lot more convenient to just rip off the PIC.
Yes, the PICs don't have a register file at all. They also have quite a simple instruction set, again providing gate savings, but then the size of program ROM may be a bit larger as a result.
The ALU can be a reasonable chunk of the processor size, and an 8-bit ALU is going to be much smaller than a RISCV ALU. Although I read somewhere that some of the Z80s, although an 8-bit processor, had a 4-bit ALU, and also I read somewhere that the 32-bit NIOS processor has a 16-bit ALU. But whether that's true or not ...
I designed a size optimised 16-bit MSP430 clone for small size low cost machxo3 FPGAs that used an 8-bit ALU. It a good way of keeping the number of LUTs down when optimising for size over speed.
For something like the low cost ice40 FPGAs, a PIC would probably be a very good match for those too compared to RISCV, because ice40 doesn't have distributed memory, which is what you'd like for register files (otherwise I expect one of the block RAMs would be used for the register file, and ideally you wouldn't want that to happen).