I would like to ask a question to customer: How can it be profitable ?

I was looking for a 15th/s which cost $6400 and $0.0015/gh/s a day.
So basically we have 6400$+ 12x657$ for one year : $14500 a year.

So if i use coinwars calculator http://www.coinwarz.com/calculators/bitcoin-mining-calculator/?h=15000&p=0&pc=0&pf=0.00&d=47427554950.64830000&r=25.00000000&er=288.01000000&hc=6400
With constant diff and constant current price of btc/usd it would generate 16720$ a year which mean only 2220$ of profit a year.

So it would take 3 years to just return on the setup fee (6400$).

But this calculator is wrong because difficulty is raising with time, and generated btc is lowering with time for constant hashrate.

if i use https://bitcoinwisdom.com/bitcoin/calculator with a 20% raise of difficulty, it's never profitable.

So what am i missing here ? and what can make it profitable ?