moving from spinning rust to SSD was a huge improvement in day to day life. i still see lag (on my very fast machines) opening programs or loading data on my laptop that would be highly improved by doubling the speed of the hard drive. and i’m not even counting compiling or things that we might do that is different than other users.

That's because SSDs had way lower latency. Upping the pcie gen will speed up sequential reads which were already really fast, but won't speed up random reads (or reduce latency) at all.

It could allow for more parallelism (deeper queue depth) with those reads though

Right but nvme are at the level of speed where you need to optimize apps for it else they get CPU-bound. Difference between mid and high-end NVMe for most user stuff ought to be minuscule