They are lost for good. Think of it like someone losing their wallet in the ocean. It seems like it would be a problem but it's really not that significant. People lose their gold jewelry all the time but it doesn't make much of a difference to the value of gold.
But loosing 1 bitcoin in the ocean of a maximum of 21.000.000 coins seems significant to me, so not comparable to gold and jewelry.
So only 21000 losses of 1000 coins can happen. That's not much!