The AMD compute dies are only connected to the IO die and other compute dies (and power). All IO connections exclusively go through the IO die, so the IO die can be customized to change the IO of the CPU without changing anything about the compute dies. It would be entirely feasible to just re-spin the IO die to add support for different memory, Thunderbolt or other IO ports. The IO die is also made on a cheaper, lower-density and performance process (14 nm / 16 nm) than the compute dies (7 nm).
