For the remainder of BTC surplus, miners harvest nodes to acquire coins... After the 21,000,000 are mined additional amounts come from transactions over the entire Bitcoin network.
My understanding is that once the 21 million coins are mined by 2140, there will be no block reward as such. The miners will get only the transaction fee, which can vary. Anyway, I don't know whether BTC will survive up to 2140.
It might. Untill then people now are very early adopters still. There only a few bitcoins so far and only so many can be mined. As more people adopt and in ten years time if you have saved a few whole bitcoins you might actually end up being rather wealthy. Lots horse our coins and wait and find out. ^^ Maybe by 2140 another Bitcoin is out and we do a swap xP