Why does mining profitability tend towards zero? Why does mining profitability tend towards zero?

Is it different for short term miners than it is for long term miners?


  Example: if I mine now and sell them next year, they may be worth more as next year they'll be harder to mine


Thanks
As with all things related to supply and demand, the market will eventually find its equilibrium, where the cost of generating the Bitoins will be close to their sale price. If the costs were bigger, less cost efficient miners would not mine. If the price was higher, there would be more room for investment that would increase the difficulty, thus increasing the cost to mine every Bitcoin.

If you want to calculate how profitable mining is, just put the data into any Profitability Calculator.