A lot of the rising price of used cars is due to the rising price of new cars,
Citation needed?
It's certainly the case that
some new cars are increasing in price (though this doesn't fit with historical data on inflation adjusted car prices, I'll grant you
can spend more - inflation adjusted - in 2020) but the value you get for economy new cars in 2020 is astronomical compared to that of 20/30/40/50 years ago.
And even though prices of cars aren't massively increasing the value you get from the vehicles is considerably higher.
There's tons of articles easily findable talking about the inflation adjusted price of cars not rising which refute this underlying premise.
And this is all ignoring the fact that buying a car in 1970 and expecting it to go 100k miles basically no matter what would have been insane. Cars have not only been constant or decreased in inflation adjusted cost but the quality of those vehicles has gone up astronomically.
In fact, it's so much less what your whole argument is about and more that the quality is high. If a car usable lifespan goes from 50k to 150k miles... the depreciation curve is basically 1/3 as much.
So naturally as vehicles last longer, they depreciate less, meaning value is lost more slowly. Why would a car that lasts so much longer depreciate as quickly? It doesn't.
If you figure a new car at $25k in 2020 dollars has a lifespan that goes from 100k miles to 200k miles and depreciation reflects this, after 100k miles - how much will the first car be worth? Basically $0. But the second car might be worth $10k or something because it's only 1/2 used up.
You can pretty easily see this, just pick a fairly common vehicle and put in new prices and then find used ones over time. Plot them vs each other - you will see that the trend has the "$0 point" way past what it would have been historically. We bought new last time since you had to drive 200k miles before the new car depreciation curve was steeper than equivalent 3-4 year used vehicles.
tldr: This assumption here is just not true.