How sure are you that the rest of the coins are actually missing and that all those people have lost their private keys? While I doubt the number that you put there (5 million coins), I also doubt that these coins are all lost. It's possible that these addresses have owners that are just waiting for the right time to sell their coins. Just because a coin is sleeping doesn't mean it's dead. It's possible that they have another address that they use daily to buy/sell things and have their thousands of coins or hundreds of coins located in an offline wallet and the wallet seed is with them. It's not that hard to believe that there will be addresses that will wake up in the future. It's not hard to believe an address from 2012 will resurrect back to life and sell thousands of coins. We should expect this to happen because as the price is increasing, it becomes more and more tempting to sell your coins.
Another thing, there might be just 21 million coins but these coins can be divided into extremely smaller portions and I think that's more than enough coins for all of us.