Of course the life cycle is important. You can't count the cost of making a new ev but neglect the cost of making a new petrol/diesel just because you, personally have an old petrol car that is holding up... for now. You make a good point that the cost of making vehicles is high and we should keep old vehicles going while they're in good working order. But if you were to change, your car that is doing fine should be passed to another user, whose own car may have given up the ghost. Thus, one new car is required: one ICE shuffles off this mortal coil and is replaced by an ev with low carbon emissions rather than another gas guzzler.
We're talking about systemic change.