The hole in your theory is that a rational miner may think the price will multiply by 100 (for example), so buying $1000 mining equipment will be profitable because it will yield $700 worth of BTC (70% figure estimated from nothing and is really an arbitrary number as long as it is less than 100%), which will eventually be $70,000. The problem is that buying $1000 worth of bitcoins would yeild $100,000 in a $1000 investment rather than $70,000 in a $1000 investment.

The real reasons people continue to mine are one or more of these: -It is fun -It secures the network -They wrongly think they will make a profit