Because the market pays less for DDR4 than for HBM (or DDR5), and since HBM is heavily modified, vertically stacked DRAM, it competes for the same raw inputs and fab space than DDR4 used.

If I can produce DDR4 for modest profit or HBM for a lot more profit I will obviously produce HBM. And given physical realities producing HBM takes from existing DDR4 production capacity. Worse still, it takes roughly 3GB of ram to produce 1GB of hbm iirc.

> People will buy it because people still have DDR4 hardware

The question is whether there’s enough meaningful demand for aftermarket DDR4 upgrades to make it worthwhile to a manufacturer to keep producing DDR4 instead of switching to HBM and DDR5.

Micron claimed retail is a rounding error, a market not worth serving. So you’d presumably need to find industrial buyers who would be willing to buy DDR4.

Basic capitalism logic is that if you think it's stupid, you put your money where your mouth is, set up a DRAM fab, and get rich.