Miners increase available supply to increase _exclusively_ as long as we are below 21 million coins. As mentioned, mining rewards will reach 0 after a few years . because of the way bitcoin works, overall coin creation speed won't change regardless of power of mining.
How many years is a few years?
It halfs ever 210,000 blocks, which is roughly 4 years.
This is a nice countdown ^^
http://www.bitcoinblockhalf.com/you can easily see how even in just 40 years time you'll half 5 times. In 2058 each block will be Worth little more than a hundreth of today (about 0.2 BTC), in 2140 bitcoin reaches asymtotic stagnation.