Hello,
if there are only 21million BTC ever, then I can't imagine, that this system will last very long! Since people loose, lost and will loose their wallets, the quantity of coins will decrease continually until there are so few coins left, that the system cannot work anymore! Where am I wrong?

Do not worry. I'll keep 1 BTC for such an eventuality. Then the world economy can buy it from me use it to run itself.