The coins in that wallet can only be stolen if the owner of such wallet misplaced his private keys. Those wallets and the coins in them could actually belong to the original owners who might have decided to buy some bitcoins and hold for that long. This could explain why their are no activities recorded on that wallet. Anything aside this reason I stated above could mean the private keys has been misplaced by the original owner. Cracking a private key is not easy and can be very difficult to achieve, it is only a hacker who has access to your wallet and private details that can crack your private keys easily.
The security of you bitcoins depends solely on how you manage your wallet, if you are careless with it, you will lose your coins and this will not in anyway lead to an end to the BTC era.
Yes you are right that bitcoin can only be stolen if owner loses their private keys. It is possible that bitcoin in old wallets belongs to people who bought it and just left it there which is why there is no activity. If something else happens to bitcoin it's likely because owner lost their private keys. It's very hard for someone to crack your private keys unless they have access to your personal information and wallet. If you are not careful with your wallet you might lose your bitcoin but that does not mean bitcoin will disappear altogether. It is important to be careful with your wallet to keep your bitcoin safe. Because not you keys not your Bitcoin.