The amount of BTC that gets "mined" when a block is created is orthogonal to the difficulty of the proof of work.
Blocks will continued to be mined at a constant rate (in mainline BTC, 1 per 10 minutes) indefinitely. The proof-of-work required for the next block to be considered valid--basically, how small a percentage of the hash search-space your hashed block has to land within--is based on the difference in timestamps of the blocks that came recently before it; effectively, the faster blocks make it into the chain, the harder it becomes to mine another, and the slower blocks make it in, the easier it becomes to mine another.
On the other hand, the BTC that the miner gets from a block is basically an implicit transaction included in the block, and acts just like any other one signed inside the block, transferring a given amount of money "out of thin air" into the account of the miner. Like any other transaction, the validity of this transaction for any given amount is a matter of policy set by the consensus of the network of bitcoin clients. The policy on the BTC network for how much money is valid to claim in an "out of thin air" transaction is a function of the block's timestamp/blockchain depth; as more blocks get mined over time, the amount of money it's valid to claim in the sourceless transaction in your block diminishes to zero. These implicit transactions will still, technically, be happening every time a block is created--they'll just be for a below-quantum (1 satoshi) amount of BTC, and thus discarded in accounting.
In the distant future, blocks will still be being mined one-per-ten-minutes, but their timestamp/block-id will translate into them no longer having any implicit value. They will, however, still have the explicit value of the transaction fees invested into them by the transactions they sign.
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The amount of BTC that gets "mined" when a block is created is orthogonal to the difficulty of the proof of work.
Blocks will continued to be mined at a constant rate (in mainline BTC, 1 per 10 minutes) indefinitely. The proof-of-work required for the next block to be considered valid--basically, how small a percentage of the hash search-space your hashed block has to land within--is based on the difference in timestamps of the blocks that came recently before it; effectively, the faster blocks make it into the chain, the harder it becomes to mine another, and the slower blocks make it in, the easier it becomes to mine another.
On the other hand, the BTC that the miner gets from a block is basically an implicit transaction included in the block, and acts just like any other one signed inside the block, transferring a given amount of money "out of thin air" into the account of the miner. Like any other transaction, the validity of this transaction for any given amount is a matter of policy set by the consensus of the network of bitcoin clients. The policy on the BTC network for how much money is valid to claim in an "out of thin air" transaction is a function of the block's timestamp/blockchain depth; as more blocks get mined over time, the amount of money it's valid to claim in the sourceless transaction in your block diminishes to zero. These implicit transactions will still, technically, be happening every time a block is created--they'll just be for a below-quantum (1 satoshi) amount of BTC, and thus discarded in accounting.
In the distant future, blocks will still be being mined one-per-ten-minutes, but their timestamp/block-id will translate into them no longer having any implicit value. They will, however, still have the explicit value of the transaction fees invested into them by the transactions they sign.