Bitcoin miners are paid with a combination of newly issued units of currency and the transaction fees for the transactions the miner includes in the block. The system is designed to initially reward miners with a large amount of newly issued bitcoin, because it started when bitcoin was not worth very much relative to other forms of money. Every four years worth of blocks, the amount of newly issued bitcoin awarded to miners is cut by half. This is how the system guarantees that there will only be 21 million units of currency. Over time, the value of bitcoin goes up, and the system bootstraps itself, steadily shifting the mining incentive away from newly issued bitcoin and toward transaction fees.
Mining difficulty is adjusted totally differently. Every two weeks worth of blocks, mining difficulty is adjusted so that the average time between blocks remains at ten minutes. Difficulty is adjusted either up or down as needed, with limits on how large the change can be. This means that there will always be miners, because as difficulty rises some may drop out, but then when difficulty drops new miners come in. This has resulted in an incredibly strong bitcoin mining ecosystem where competition drives the development of more and more efficient ways of mining bitcoin with as little power as possible. This bitcoin mining ecosystem has been running nonstop since the first block was mined in 2009, and there is no reason to expect it ever to stop.