How do you interpret these lines from A Moveable Feast?

“I remember how they used to burn out their brakes going down the mountain roads with a full load of wounded and braking in low and finally using the reverse, and how the last ones were driven over the mountainside empty, so they could be replaced by big Fiats with a good H-shift and metal-to-metal brakes.”

It is about the Stein’s Model-T Ford and how that sort of car was converted into an an ambulance during wartime. The final line confuses me. He says many of them were driven over the mountainside empty in order to be replaced by superior cars.

Why drive them over the mountainside empty?

I also wonder if this isn’t an implicit attack on Stein, who is represented by the older sort of car (it is her exact car in the narrative), whereas he is the newer sort of car that outmodes the older one. It is evident that Hemingway has soured on her by now.

by Formiddabledrip