they’ll need to use twice as many chips, which could mean doubling the bus width (one can dream) to 512 bit (ala 5900)
I misread this part, thinking you implied a bus width increase is necessary.
For a 512 bit memory bus, AMD would either have to use 1+8 dies if they follow the 7900XTX scheme or have a monolithic behemoth like GB102. The former would have increased power draw but lower manufacturing costs, while the latter is more power efficient and more prone to defects as it's getting close to the aperture size limit.
I'd guess nvidia will soon have to switch to chiplet based GPUs. Maybe AMD stopped (for now?) because not their whole product stack was using chiplet based designs so they had way less flexibility with allocation and binning than with ryzen chiplets.
Ööö (uhhh), not really.