Forgotten or lost wallets will be a problem as long as people won't care about their bitcoins equal to their real life money. I know you can't put your hard dirve into a safe box (but a backup hard drive can be put into it). Also, people have to learn how to deal with bitcoin. Dealing with fiat money is really simple, you learn it from your parents, or from other people, but you can't learn to deal with bitcoin from the family and you don't learn it in the school. Those lost bitcoins were lost when the technology was really new and people didn't care about them much, because it had much less value. If the bitcoin price was $1,000 or more that time people would care about their bitcoins much more.