That's understood, and it was a smart move to couple SD cards to PCI-E lines.
It's just that the producers want to make money from new tech, and thus sell at a high price, especially early on. It will take years until the competition has heated up enough to bring the prices down to a level where SD Express will become a viable choice over the older UHS standards. And like I said, even there UHS-I is still by far and wide the most widespread despite the tech being 10 years old by now and UHS-II being just a year younger yet still very rare. At least the pre-UHS models are slowly dying out in industrialized countries (still saw tons of 2-8GB SDHC cards in the Philippines, so in developing countries, it's still another matter), so they are slowly moving forward.
Yup, but I do think there's nothing really that stops Nintendo from going to their cartridge supplier and saying "hey we'd like something similar for our next-gen carts, PCIe 4.0 type speed".
That probably is fairly doable for a product a few years down the line.
SD Card commission wants to move at a snail's pace, that's their call I guess, but Nintendo can still show that to their cartridge supplier and say they want something in that performance range.