Different functions scale differently. With the latest processes, logic scales best, while analog scales worse: SRAM (which is internally pretty analog) still s

Different functions scale differently. With the latest processes, logic scales best, while analog scales worse: SRAM (which is internally pretty analog) still scales decently, but less than logic; and I/O scales very little at all (think of it this way: the size of transistors that drive outputs is pretty much determined by the current you need to drive, and the current is determined by e.g. the PCIe spec, which is itself subject to the physical constraints of relatively long wires). As a consequence, if your design has CPU dies and I/O dies, using a smaller process only for the CPU dies is likely to be a good trade-off.