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I'm pretty sure the effort (processing power) required to mine a bitcoin is independent of the history of mining, and depends exclusively on how much processing power is currently being spent trying to produce bitcoin. Unless I've drastically misunderstood the algorithm for difficulty, which I'm relatively sure I haven't (thinking of this page specifically).
The consequences of this seem obvious to me; bitcoin is only difficult to acquire when it is perceived as worth acquiring, meaning that a loss of confidence that it has value leads directly to it being much easier to acquire even without having to purchase it from people who have lost confidence.
The bitcoin supply is limited by design and as you approach the last bitcoin which could be mined, the effort needed increases. That makes the effort needed dependent on the "history of mining", or, more precisely, on how many bitcoins have already been mined.