Asking price means almost nothing. You need to look at actual selling prices to make any conclusions about retaining value.
About 7 years ago I did extensive analysis of pricing history of a major US manufacturers' boat of many models ranging from 28 to 36 feet. I used Soldboat.com data, which admittedly may create selection bias by focusing on the best maintained boats. I found that, for the most part, boats in the 5-15 year age range sold for about their original purchase price when new. The flat selling price over time meant depreciation, due to owners' loss of opportunity cost (vs. putting their dollars into growth investments), inflation, non-recouped maintenance costs, and all the owner upgrades that did not increase the selling price of the boat. Newer boats sold for higher prices, but they were purchased for higher prices too.
Agreed with the scarcity of viable used boats. There are lots of used boats out there, but not many that you can sail away without mods and repairs. That situation will grow as the market ages and less new boats come on.
So I think the number of used boats is in decline if you discount the ones that require half or more of their value in refits.
I agree in general, especially when you see the fake wood laminates, foam-backed PVC headliners (which will lose their plasticizer and crack in about 15 years) and other cheapening features of some newer boats.
In both of the keel boats that I have bought, I have paid a few thousand dollars premium for better than average condition, with the expectation that repairs and reliability would be better, which is critically important due to the cost of losing sailing time if something breaks. An old, worn boat can be far more costly to keep going in the long run.
I keep hearing these stories of old cheap boats that "just need a little TLC to triple their value. I see a lot of them in boat yards, not sailing. Haven't really seen one yet where the owner was able to turn it around quickly and cheaply into a much more valuable vessel. You can pour a lot of time, energy, and materials into it and still end up with a boat that was worth about what you paid for it.