>Even fast SSDs can get away with half of their lanes and you’ll never notice except in rare cases of sustained large file transfers

Sorta yes but kinda the other way around — you’ll mostly notice in short high burst of I/O. This is mostly the case for people who use them to run remote mounted VM.

Nowadays all nvme have a cache on board (ddr3 memory is common), which is how they manage to keep up with high speed. However once you exhaust the cache speeds drop dramatically.

But your point is valid that very few people actually notice a difference

You're pretty far off the mark about SSD caching. A majority of consumer SSDs are now DRAMless, and still can exceed PCIe 4.0 x4 bandwidth for sequential transfers. Only a seriously outdated SSD would still be using DDR3; good ones should be using LPDDR4 or maybe DDR4. And when a SSD does have DRAM, it isn't there for the sake of caching your data, it's for caching the driver's internal metadata that tracks the mapping of logical block addresses to physical NAND flash pages.