>>> then a defect in one CPU block would wipe out an Intel Chip while leaving 3 of the 4 chiplets cut from the same section of wafer usable.
that isn't really how it works, Intel's large chips don't need to be defect free to be usable, they can disable bad cores and cache which are the largest parts of the die (AMD also does this) plus their Xeon line up has dozens of SKUs that come from three different core count die: 10, 18 and 28 core die sized from 322mm*2 to 688mm*2.
>>> In summary, I would guess that the large I/O chiplet actually costs AMD less than the small CPU chiplet. Admittedly speculation based on general semiconductor industry info since I have not seen any true AMD cost data.
i'm not so sure...we would need lots more data since AMD gets to use the smaller 80mm*2 CPU die even if they yield less than 8 cores while with the larger 400mm*2 I/O die I'm not sure what opportunity they have to work around any defects.