Yeah the cheapest time to buy old tech is always just when the new stuff has come out. That's when suppliers are trying to shift old stock at cheaper margins.
You can take a look at the 5800X3D and how it was at its cheapest about 2 years ago when AMD was winding down production and Zen 4 had been launched.
To further your point, a used 5800X3D still goes for ~$300 when you can get a brand new 7800X3D for the same if not slightly cheaper (was on sale at MC for $280). I assume the high cost of DDR5 is pushing more people to not upgrade to AM5 and stay on AM4 for as long as possible. And most people are avoiding Intel chips out of principle after the chip degradation debacle - you can see that based on how much lower Intel motherboards are even though they have considerably better feature sets than the AMD equivalents.
Main reason for Intel boards being cheap is because they're practically one and done because of Intel's insistence on changing sockets every other generation.
You can take a look at the 5800X3D and how it was at its cheapest about 2 years ago when AMD was winding down production and Zen 4 had been launched.