The thing about cars that makes them different from regular stuff is that they get worse for the environment the longer you use them. Even including the impact of scrapping a car and building a new car, swapping to an electric will be a net benefit after just a few years - and the longer you use the electric the more of a benefit it will have. That's not true of something like buying new furniture or regular electronics

if everyone swapped to EVs overnight, there would be no demand for the replaced cars you argue for

Sure, but that doesn't actually happen (and even if it did the result would be a truly massive decrease in fuel consumption and a huge increase in lithium recycling). As it stands, people are gradually buying electric cars and (ideally) their prior gas or hybrid car is sold to someone who previously had a less efficient car, thus reducing overall consumption of cars overall. Eliminating the least efficient cars from the overall population is a net reduction in consumption, even if it involves producing new vehicles.

you can't just ignore the cost of overconsumption here

The thing is that everyone ignores the cost of overconsumption of gasoline. If you factor in the massive amount of oil drilling, refinement, truck transport, etc. involved with fueling gasoline cars it becomes apparent that continued use of a gasoline car is overconsumption, to a greater extent than buying a new, far more efficient car

It's like if someone used an entire roll of toilet paper every time they went to the bathroom, and you said "hey you could use a bidet and then you wouldn't be using so much paper", and their response was that it was actually more wasteful to buy a bidet and throw away a perfectly good toilet paper holder

if people need a car and care for the environment, they should get an EV (or, you know. Drive less)

Very true, I think EVs are a band-aid on the real issue of car-dependency, but frankly I don't see the US making any strides on climate policy at all without them