Hey Umberto,

As the Bitcoin given to miners for securing Bitcoin transfers decreases, Bitcoin plans to start taking a percent or constant value of the actual transaction (from the client).

Just as an example, say that if a miner secured a 500 Bitcoin transfer, they would get 12.5 Bitcoin. When the reward becomes very small, a miner will be allowed to take, say, 5 Bitcoin from the transfer. Hence, the person on the receiving end of the transfer would then only receive 495 Bitcoin, which is obviously undesirable but a commission that many other payment transferring services take.

Bitcoin will never run out of miners, for the simple reason that if everyone leaves because the reward is too small, there will always be someone who looks at the small but very abundant reward and does the work to reap the benefits. A small amount multiplied by a very large quantity is a decent amount of Bitcoin.

Cheers!