Remember that 21 million is a completely arbitrary number. Bitcoin would work as designed even if there were only 1 bitcoin. Right now we can divide BTC out to 8 decimal places. If we moved to 9 we move an order of magnitude. This is easy to do and solves any shortage of bitcoin supply.
As mentioned above by several posters, any attempt to "re-mine" those coins should be outright rejected. Who will determine what a "lost" coin is? How would you know if my coins are lost or in cold storage? After you re-mine coins then someone shows up and says "I found my coins!", who's coins are now the real coins? Are you going to replace the found coins, and with what coins are you replacing them? And under what authority can you render some coins unusable?
If the 21M cap is ever lifted I would divest at least 50% of my BTC and I would assume the project is now dead. A strict, unchangeable cap is one of the most powerful features of BTC. Without it you basically own benie babies or tulips.