@locus-solus: NAND flash is actually getting quite cheap to manufacture. Now it only costs about 10 cents per GB, or 10 GB per dollar, to manufacture. For $5, they could manufacture the NAND flash equivalent of a 50 GB Blu-Ray.
As NAND flash prices further decrease (as per Moore's law), optical discs will become obsolete and NAND flash will dominate. Which means the potential comeback of cartridges making optical discs obsolete, ironically. Optical discs once made carts obsolete, and now it looks like carts may end up making optical discs obsolete in the near future.
While Nintendo paid the price for choosing carts over CDs with the N64, Nintendo this time made a smart choice picking flash carts over discs for the Switch. Flash memory technology is advancing to the point where carts would no longer be much more expensive than discs like they used to be.
$5 equivalent for a blu-ray that surely costs <$1 per disc pressed at scale. That's actually still a massive difference. 5x cost difference. This is why it seems Sony is going with optical, and they also want to help push the last gasp of optical formats with UHD media. Publishers love optical media because pressing out additional copies is almost a negligible cost.
That "cartridge based" might happen next-next gen (PS6), but by then it might be a moot point as the market is going more and more digital. I do see maybe that as a possibility instead of a 100% "digital only" console they will go to 128GB cartridges which by then will probably be as cheap as optical disc. If there was price parity there would be no question to go with faster NAND memory.