But now you're dealing with probabilities, weighed against the certainty of the original EV purchaser emitting less.
Try this: if everyone in the US suddenly purchased EVs, and ditched their ICE cars, flooding the market with old ICE vehicles, would emissions decrease in the world or increase? I think it's pretty clear that the vast majority of the old ICE vehicles would be junked, and there'd be marginally more vehicle-miles-travelled, so the huge wins of everyone using EVs would counteract any increase in vehicle miles from suddenly having cheaper ICE available around the world.
So I would argue that the single person doing that action would have the general same trend as if everyone did it.
Try this: if everyone in the US suddenly purchased EVs, and ditched their ICE cars, flooding the market with old ICE vehicles, would emissions decrease in the world or increase? I think it's pretty clear that the vast majority of the old ICE vehicles would be junked, and there'd be marginally more vehicle-miles-travelled, so the huge wins of everyone using EVs would counteract any increase in vehicle miles from suddenly having cheaper ICE available around the world.
So I would argue that the single person doing that action would have the general same trend as if everyone did it.