Though I would point out here that at some point in the future we may find that we need to upgrade cryptosystems for security reasons... and that if we don't _eventually_ deny the spending of coins sent to the old insecure system then we could well find that enormous deposits of zombie coins suddenly appear due to cracking and that their rapid unexpected introduction into the economy is very much not what anyone signed up for and could be disruptive. So it would make sense to sunset insecure transactions.
I see yuur point, but you are not entirely on topic.
The topic author is not suggesting to "sunset" coins for security reasons, but for some bullshit semi-inflational reasons, which is plainly wrong and destroys the sancrosanct rules of Bitcoin as Gavin said.
Once the cryptography behind Bitcoin gets broken, I am sure that majority will agree that something needs to be done about old, insecure coins. In that case, i suppose it would be possible to re-hash the old coins somehow with new algorithm by sending them to a upgraded address using upgraded client.
But that is a matter for completely different discussion.