Would you be able to point out at what USD/BTC exchange rate it will net you more profit to mine say 60 BTC from a piece of hardware that cost 65 BTC to purchase than it would to just hold onto the 65 BTC?
The point you're missing is the fact there needs to be two chronologically separated exchange rates for this
So lets run some numbers. Lets also use some easy numbers and assume a 100 times increase in exchange rate from time of purchase to time of sale of coins, sound good?
Scenario 1:
Customer has (or buys) 65 BTC each valued at $1.00 USD each. This customer now places an order with a hardware manufacture and buys a xxx GH/s miner. He has invested 65 BTC at $1.00 each for an initial investment of $65.00 USD. Now when the miner is no longer profitable the total number of BTC the unit mined was 60 BTC but in the meantime BTC has increased in value 100 times and each coin is now worth $100.00 USD. You sell all the coins and have in hand $6000 USD wow, awesome solid investment right?
Lets look at Scenario 2 to see how we would have faired just holding BTC
Customer has (or buys) 65 BTC each valued at $1.00 USD, initial investment is now $65.00. At a certain time in the future (for arguments sake lets say the same amount of time it takes to mine 60 BTC with a miner you could have purchased) BTC increases in value 100 times and now is the time to sell. Customer sells all 65 BTC at $100.00 USD and has in hand now $6500 USD.
Feel free to plug in any values (different, same, any old number at all it won't make a difference) for the exchange rate at the time of purchase and the time of sale. Two chronologically separated exchange rates doesn't change the maths involved at all unfortunately.