When you'll already have the 21 million bitcoins, but the userbase is still growing or the mined coins are lost, happens deflation. The prices of the goods and services will drop, because each of us will have less coins to spend in the long run.
The user base has nothing to do with inflation or deflation. The inflation or deflation is price of money measured in itself if the factor of time is included. The price of goods and services will drop, because labor productivity is increasing.
Forcing inflation into the monetary system is equal to forcing people buy goods and services they don't actually need! This is not economic growth. This is economic disaster. Because such a monetary system gives wrong signals to the rest of the economy including investors, manufacturers and consumers.