So here is a worse case scenario, that EVERYONE who installed Bitcoin, in an unprotected fashion, which would probably be most people. may have had their wallet 'peeked', you might not have cared then, you might not care now because you only have a few Bitcoins, but theoretically, hackers could have been scouring the net, looking for unprotected wallets and taken copies.
In 5 yrs time, many of these wallets may no longer exist, but a few at least may well have significant amounts of Bitcoins in them, which they wil be able to use because they have your wallet, which is more accurately likened to a Key.
The part that you are missing is that a backup is only good for a certain number of transactions.
The wallet contains a pre-generated number of addresses. When you copy,or back-up, your wallet those addresses go with it.
Once the number of transactions goes beyond that prefixed number then the old backup is worthless. because the key that goes along with it is now out of date.
-Jason