> Different types of DRAM can literally be made from the same already-etched wafer.

The assumption here is that you would stop making DDR5 but continue to make DDR4 so that you could start making DDR5 again without too much trouble. But the older chips have even lower margins than the newer ones. Most of the fabs and equipment for making DDR4 were created when it was current and then they stay in operation as long as there is still enough demand for it.

If you don't make DDR5 and DDR6, what happens to your DDR4 fabs when DDR4 is where DDR2 is now? They close because nobody wants it anymore. And then you're not trying to get to DDR6 from DDR4, you're trying to get to DDR6 from an empty desert.