If externalities were correctly priced in to fuel, rare earths, rubber, road wear etc then it would be easy to see, the cheaper the better.
But they aren’t, not even close. Oil is massively subisidised by the military before the environmental costs. Brake particulates and tyres don’t cover the cost of microplastics and lung damage, heavy cars don’t pay anywhere near the damage they cause to the roads and bridges etc.
Due to this you can argue pretty much whatever you want by ignoring certain costs depending what you want to come out with.
My petrol car is 20 years old, it’s done 70,000 miles, it weighs about 1,000kg and burns through 300 litres of unleaded each year to do the 3,000 miles I do in it.
I suspect scrapping and replacing this with even a small electric car would not be globally environmentally worthwhile. There may be improvements to local air quality assuming regenerative breaking etc, that may be offset by increased tyre and road wear though, even ignoring the impact of the co2 to generate the 80kWh a year it would require.
20 year old cars tend to be heavy polluters because they don't meet the latest emissions standards. Here in California the state will buy old cars and scrap them to get dirty emitters out of service. Also, nearly every day electrical generation is over 50% using solar, wind or hydro so EVs are cleaner here than any ICE vehicle by far.
Well my 20 year old car meets the various clean air zone emission standards that newer cars fail to
However even if it didn’t, if I used it for 200 miles a year would it make sense to buy a new electric car?
It’s never clear cut, and it’s practically impossible to make the best decision in any given case. You can make a regulation which will on aggregate lead to less damage but there will always be exceptions, and on a case by case basis it’s extremely difficult to measure the damage a given scenario applies. How many “units of badness” does buying a new 2 ton electric car before you move it a single mile. Id wager it’s more than an existing petrol car burning 1 litre of unleaded petrol on existing tyres and brake discs.
The difficulty is measuring total impact of the choice. Sure buying a new petrol car and driving 20k miles a year for 6 years will be worse than buying a new electric car and driving 20k miles a year for 6 years. That’s not where the line is.