It is infinitly divisible (up to 8 decimal places right now, easily expandable with a modification of the client); so even if at one point there is only a total of 1BTC circulating in the whole world that will still be more than enough
There are issues with that scenario that I have pointed out in other threads, but others don't seem to agree with me. Assuming the scenario you have illustrated, you must agree that it is possible that 100, or even 1,000 Bitcoins could be rediscovered, thus the World's economy, which was thriving on 1 Bitcoin, has now undergone 100 or even 1,000 fold inflation overnight if the Bitcoins are dumped on the market.
As people find deposits of precious metals and minerals today? The world doesn't end. Some people just get rich.