Also they would need to adjust the refund it with inflation.

Plus interest

Plus interest makes no sense and is unnessary. Adjusting for inflation is so the person can buy the same movie at today's prices. At that point, the person is made more than whole already because they got to see the movie for free and they can chose to get it again.

If they are able to prove it somehow transparently that movie was watched at least once, reduce the amount of renting it based on the time when it was bought. Is that fair? Return the rest with inflation adjustments.

When you buy an option, you don't deserve a refund for not exercising the option.

On the other side, a user that watched a movie 1000 times should not get less compensation.

All users are being impacted exactly the same way (loss of future access), so should be compensated the same way.

No it’s not fair.

They've stolen the property. It’s not “fair” if you replace it, let alone only replace it with a lesser than value as “it was second hand”.

The interest and inflation are the time value of money, which you pay, for the time value of access.

Original-price refund is both simple, and (by linking the time-value of money for both parties) fair.