If no one came and you waited for weeks, months, then what's wrong if you consider that from now on, it belongs to you? You haven't stolen from someone. Someone lost it, you tried your best to find the owner but owner did least to find you, then it belongs to you. You push yourself too much, you think like you are the killer if you tried to assist wounded person but failed to escape him.
The issue with this line of thought has been pointed out in this thread, before. In essence, the reason it is immoral is because there are a ton of scenarios where the actual owner has another copy of the keys and expects the money to still be there when they try to use those keys in the future.
For instance, this paper wallet may be one of a few backups and they may not check the integrity of those backups regularly.