Since it has been a while and we are kind of off track again, here is a summary of the actual proposal by Theymos (yes he did join this thread briefly - you can see his comments by searching the thread):
When/If QC becomes a reality (this is a big When/If) and
After QC becomes a viable threat to Bitcoin and
After we have already replaced the QC vulnerable algorithms in Bitcoin with QC safe algorithms which means
After a highly publicized, bitter, drawn out, drama about the "death of Bitcoin" because of QC in every news outlet everywhere and
After a majority of new addresses and transactions are using the new QC safe algorithms which means
After a vast majority of all Bitcoin users have heard about the issue, the solution and the consequences of not moving their coins
Then and only then, maybe, we should consider possibly blacklisting older coins that are not covered by the QC safe replacement algorithms, especially the more highly vulnerable very early coins.
This only makes sense after we have transitioned to new QC safe algorithms
Note that at the rate QC is progressing those of us alive right now really don't have to worry about this. Perhaps our grandchildren or maybe our children.
Now my opinion. Yes I have one too.
Assuming that most of the 21M coins have been mined by the time this happens and assuming there are about 1M highly vulnerable coins in the blockchain then don't worry about it at all. In other words just let those 1M coins, the most highly vulnerable coins, the easy pickings, be stolen by the people with the QC. The worst case scenario is that the brilliant people who have access to the most advanced technology are also the stupidest people or are a nefarious actor and they dump the entire 1M coins on the open market. Two things will happen:
1) The price will go into the toilet for a while, those that don't panic will get to buy up a bunch of cheap coins and make a fortune when the price recovers, those that panic will lose everything. In other words
business as usual in Bitcoin land.2) We will know this has happened as soon as it happens so everyone that has their coins on the remaining vulnerable addresses will immediately move their coins to the new QC safe addresses - problem solved.