Isn't morally incorrect to not utilize money that might be lost? If you knew that it's likely for the owner to have lost access, wouldn't it be incorrect to "donate to everyone" instead of making a usage that's more morally correct, for example charity?

Let's say you never find contact with the owner. In that case:

• There's a chance for the owner to still have access, and never realize he's lost the backup. Emptying the wallet in that case would be morally incorrect.
• There's a chance for the owner to not have access, and can't provide you the necessary evidence. Burning the wallet in that case would be morally incorrect, because emptying it would have the potential to do more good than making everyone else's coins more valuable.

So, whether you burn or spend the coins, you might have a morally incorrect behavior.