If someone lose their bitcoins (whether it is small or big amount), does it have a significant impact to the bitcoin economy?
Let's think something very positive, specifically about the price and economy of bitcoin. I'm not sure it will be very significant, but each bitcoin that is lost will make other bitcoin rarer which will then make the remaining bitcoin more expensive. Of course it must be due to the demand factor, but can't conclude that it won't go down because of the volatility.
We shouldn't consider them gone for good.
Of course, something now said to be lost specifically about bitcoin could come back into circulation if its owner one day remembers his password or regains access to his wallet. I agree with you.