Over the next century as thousands of BitCoin owners will undoubtedly lose BitCoins due to disk failures or other unrecoverable problems, what will happen to the network in the event of a very large number of dead coins? Would the hypothetical complete loss of a large account (say that of an entire BTC exchange) cripple the entire world economy? If the lost coins were no longer in existence it would thereby reduce the number of coins in circulation and devastate the entire market as I can see it... But I honestly don't know on this one.
Please somebody explain the criteria by which dead coins might be replaced with active coins available to be mined.
Dead coins are not being replaced. After all, how do you determine whether coins are no longer accessible or just stored in a very long term cold storage?
Anyway, if coins are lost, there are less coins in circulation and that means the price will go up (if demand stays constant or increases). Bitcoins can be divided into 100 million smaller units, so it will take quite a while before a single satoshi is too large of a unit of value. And at that point, the divisibility of the Bitcoin can be increased through software updates.