> If there's a trade war, they can easily reconvert to making the mass-market stuff

Factories, tooling, supply chains, and engineering knowledge aren't fungible in the way they would need to be for your statement to be true.

Different types of DRAM can literally be made from the same already-etched wafer. The DRAM bits themselves don't change at all. What's different between DDR4, DDR5, and HBM is the IO interface to the chip. Changing this does not require significant retooling or relearning.