There are no supply chain issues.
Micron has cut NAND production. Samsung has cut NAND production. Kioxia has slashed production. Sales have plummeted for the last few quarters due to recession.
But for whatever reason, Apple seems to have entirely moved away from 128gb nand. And only purchasing 256gb nand. Which is what is causing this issue. So perhaps their are supply issues at apples scale for 128gb nand. Maybe they'll move next generation to 512gb (2x256) base for m3 and 1tb (4x256) base for m3 pro/max/etc.
I think the main issue is people see this synthetic benchmark of 3000 vs 6000 (on the new m2 max models, the old one were apparently more like 3900), and think oh no its half as fast - my computer will be half as fast. But its very unlikely in almost any work load on a m2 pro chip, that you would actually encounter a situation where you could observe this as an issue.
SSD speeds have really increased in the last few years. A quick look on google found this PC gamers experience of moving from a 500mbp/s SSD to a 2000mbp/s drive:
I personally used two types of SSDs. The older one was only about 480 Mb/s Write and 500 Mb/s Read. Still, an incredible speed while gaming. Now I’m using one from Adata: 1500 Mb/s Write and 2000 Mb/s Read. And I honestly can’t see the difference in loading times. Those are still great, but are they faster? Perhaps. Not by much though.
What actually matters is that the whole computer can take advantage of the drive speed, if not, the theoretical max speed of 6000 is meaningless. Which is likely the case on the m2 pro.
The cost for upgrading to 1tb is not expensive. It's not likely to be noticeable in practice. And means, they are able to offer a lower entry price point for the machines up and down their line. People act like Apple's alternative was to give away more storage for less, but, its apple, they dont really cut margins, the option for them was increase the base price, or do this.
I look forward to seeing the youtuber/website which has a real world demonstration of this effecting performance.