There's a built-in mechanism for ensuring that this is never a serious problem: the more coins that are lost, the more valuable the remaining coins become.
And the more valuable something is, the more careful people are not to lose it. So there's a nice feedback loop that should naturally limit the number of coins lost.
If Bitcoin becomes the world's stable reserve currency then one bitcoin might end up being worth tens of thousands of dollars and we'll all get much more serious about backing up our wallets and keeping them secure. Or, I think more likely, most people will use a service that they trust to keep their coins backed up and secure...