This is a good idea.

Let's look at a scenario where we've got 20 people, each with about 10k bitcoins they've acquired over 10 years. These 20 people all happened to die in the next few years (car accidents, heart attacks, cancer, whatever). That is 200k bitcoins that are instantly taken out of circulation. With a continuing trend, over enough years bitcoins will fade out and the whole communities dreams of bitcoins replacing our current currency is destroyed. At this point in time, sure.. 200k doesn't seem like a lot when there are plenty to go around because they're still being generated. Once 21m coins are reached, the amount of "lost" or forgotten bitcoins will start to add up quickly.