In a couple of years, with so many coins lost this way, not sure many will be left, since bitcoins are limited to 21 million and losing coins by sending them to wrong address's will keep on happening. Anyone can make a simple mistake copying the address, even though it's easy.
Yes, it's like with cash and washing machines. So much gets destroyed, they even need to print new notes all the time!
that doesnt have much bearing on bitcoins. notes are reprinted because old ones wear out or a design needs to be updated. that doesnt matter for bitcoins.
21million will be plenty - even with lost coins. if and when bitcoin actually takes off, 1btc will be a pretty large amount. i wouldnt be surprised if day to day transactions would be <1 btc... loaf of bread: .00001 btc, pack of smokes: .0002 btc, etc. there is plenty of room to divide a single bitcoin that lost bitcoins wont really matter.