Well obviously there are always going to be more used cars on the market, but given that well over 2 million new cars were sold and registered in 2019 not everyone thinks like you. Also yes, over the course of 10 years that would mean 20 million cars are replaced, so if you are running a 10 year old car then you rely on those people to supply your vehicle.
Some people buy a new car and keep for 8-10 years, this kind of scheme allows them to get way over value for their current car, and buy another new car that will last them just as long.
Oh sure. But that doesn't detract from the fact that the majority of people will be buying used cars, typically for a great deal less than the new price.
(I have always reckoned the "Half Life" for car values to be around three years)
There will be some that buy new and keep for that long, but that is rare. Most new car buyers are habitual new car buyers and will only keep for a relativly short time. Very few people who are running around in 10-15 year old cars will be in a position to borrow the sums required to buy a new EV. (Or any new car really)
And one very big diference between EV's and ICE cars is that when I bought my latest car, A mid 90's E36 for £500 with 150,000 miles on it, (I accept that I am likley to be at the other end of the extreme of course, it helps being a mechanic
) I am confident that it will have the same range/performance enverlope that it had when it was new.
(Possibly even better. I couldn't find the youtube clip but top gear did a drag race between two identical spec golf GTI's many years ago where the only difference was that one had low mileage on it and the other had 170,000 or so. The higher mileage car left the lower mileage one for dust)
Now, I dont know how the price of used EV's is going to hold up but I would guess that they will hold up quite well and only drop in price once they become unusable. This is quite different to the depreciation model for ICE vehicles where old and cheap options will still retain new levels of performance.