Yes, wattage is not the only factor. There are entire threads dedicated to power supply quality. However, this thread was regarding the accuracy of PSU calculators, which are suspect. But they operate on simple concepts.
While I agree that CPUs are not likely to reach maximum draw while gaming, that wasn't really listed as what the system will be used for, so we can't make that assumption. I don't know about you, but I tend to benchmark my PCs when I build them to see how they stack up.
Regardless, if you are not planning for a worst case scenario, or accounting for PSU aging over time, it could bite you later.
GPU power spikes go well beyond the average power consumption. They don't often list the OCP limits on PSUs, so you more or less have to rely on reviews or known good power supplies.
Worrying about PCIe 5.0 and ATX 3.0 isn't really worth it yet. It takes a long, long time for ATX standards to shift. There will still be a market for ATX 2.x PSUs for at least ten years. Just as there was, and still is, a market for ATX 1.x and even AT power supplies. GPUs will come with appropriate adapters and PSUs will as well for a while. Also there are limits to what I would be willing to cool in a system, and I am about there with a 350W GPU. I probably would not opt for a 450W+ GPU if I could avoid it. That will just make for a computer much larger than I would want.