If mining becomes too expensive, then people stop mining.
When people stop mining, competition for mining falls, making it easier to mine.
If it's easier to mine, mining becomes cheaper, so people start mining.
GOTO 1
When people stop mining, competition for mining falls, making it easier to mine.
If it's easier to mine, mining becomes cheaper, so people start mining.
GOTO 1
The only thing that the number of people mining directly affects is the cost for an attacker to perform a 51% attack on the network.