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.