YABU to rely on being able to do this, because the supermarket may already, or could in the future impose time limits on cars being in their car park, or lock it outside store hours.
Or they could increase the cost of charging, making it uneconomical to charge a car - there's already many commercial chargers where the cost per mile works out to be similar to that of a small engined petrol car, making running an electric car uneconomic for many.
Or even if the above doesn't apply, other people in the street could have the same idea and you could spend half your free time watching the supermarket car park, waiting for a space.
Outside certain financially comfortable bubbles, where people rarely or never have to do long, journeys in an efficient time period, we are a long way from electric cars being practical and efficient for the majority. Plus we probably don't have enough electricity capacity for a lot more people to switch to electric cars.
The long queues for chargers at service stations over Christmas were a good illustration of this.
If you have your own charger and do sufficient mileage in short to medium length chunks so you rarely or never have to charge away from home/work and normally run an expensive car, electric cars can be worthwhile and affordable. But if any of that doesn't apply, not so much right now.