I have just read this whole thread from start to end. I feel for the people who have "lost". However, new and second hand car prices are set by the people selling them based on how they think can do best. A manufacturer wants to maximise profit and that is combination of profit margin on each car sold and number of cars sold. A private seller, or garage, selling a second hand motor wants to maximise what they get for disposing of their asset. These figures will go up and down in time.
I do not not know whether it is the same in the USA, but in the UK I have a rule of thumb, that whatever car I buy, of whatever age, I expect it to be worth about half that amount three years later. This is very approximate and some cars do slightly better and some slightly worse. For the record I have never bought a brand new car, but looking at prices, in the UK that rule of thumb is still good for a brand new purchase. I (or my wife) have a 9 year old Model S which we bought three years ago. Guess what? If I wanted to sell it tomorrow, it would be worth about half what we paid for it.
To be fair, I don't think that slashing the price of a new Model S has changed the value of our nine year old example. However, in the UK at least, the price of an early Model S has been closely aligned with the price of an early Model 3 for some time. So when Tesla slashed the Price of Model 3's earlier in the year, a slice was wiped off the value of an early Model S as well.
Do I care? No. My wife loves her Model S and has no plans to change it. Happy Wife equals easy life (and vice versa).
As an aside, I work in shipping. In the 2008 credit crunch the value of ships plummeted, both new and second hand. Buyers were desperate to pull out of contracts but unless they could significant deficiencies in what they were buying they were unable to. I do not see a drop in car prices being much different. If you have just bought an S and it has a major fault then reject it. Otherwise enjoy your purchase and try not to think about the ifs and buts of buying it at a slightly different time.