i'm not going in to the technical part about privkeys and other stuff, lets assume it is possible..
so, even if it is lost, who gets these bitcoins? how to address them without conflicts?
"let's send them to charity"
ok, so, which one? cancer? africa help? some group fighting isis?
every possible winner of these bitcoins would cause conflicts with a part of the community. that would be always a reason to fight, and fight in blockchain world = fork.
If you are not quite happy with the expired bitcoins being sent to a piggy bank address, I think they could be just rendered unmined, i.e returned to the pool of yet to be mined bitcoins. In this way, there won't be any conflicts as to who should accept the free lunch. On the other hand, the owner of these coins could explicitly set the address to which they should be sent after the time comes...
That could be construed as a will expressed through the blockchain
I saw somewhere either in this thread or another that one idea to get around those issues would be to "reset" coins after a 10-year waiting period where the coins are never activated in one way or the other, and they would go back to being mineable coins for the miners to go after.
It was in the opening post