As your calculations show, it won't pay for itself outright. However it will be 5 years newer (or whatever it is) and lower miles. If you would normally keep a 75k mile car for another couple of years and shrug your shoulders about inevitable servicing and MoT bills around 100k miles (brakes, suspension, wheel bearing and if you're unlucky a clutch) then do so, but if you were thinking of a change then a good deal on an electric car is probably a good plan.