The thing that you're missing is that Bitcoin mining has a fixed reward schedule that is independent from the number of miners. If you have more miners, that just means that each miner is yielding less Bitcoin (this is accomplished by scaling the difficulty level, which is currently 1,590,896,927,258X what it was when Bitcoin was released in 2009).

The result is that the only factor that affects whether someone chooses to engage is mining is if it is profitable.