Except that EVs are uniquely situated to get cleaner after they are purchased. That's already occurring, so an EV purchased in 2013 is currently cleaner per mile than it was when it was purchased [1]. You could make a weaker argument that that might apply to FCEVs. Whereas all ICE vehicles usually need to be replaced en masse to improve emissions.

[1] https://insideevs.com/us-ev-emissions/