had a back of the envelop calculation: if the loss rate is 10% per annual, it will take 21 years to reach the 1/10 mark, which can be compensated by moving the decimal point one digit to the right. Not too bad indeed.
I seriously doubt that 10% of gold/silver/platinum bullion is lost every year...
Once people understand that bitcoin is a hard money & a commodity, they will protect it with their lives, the same as they do with normal bullion. Therefore, i think that 10% may be probable for the first few years of bitcoin adoption, and when masses understand the power of bitcoin, the loses will fall to 3% or less.
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EDIT:
Also, new, better clients will be created with built-in multiple automatic wallet backups & data redundancy (or it will be implemented in the default client). So the losing of a wallet will become less common.