Profit will going lower and lower as mining going more popular. It's so predictable..
Another reason is the fall of bitcoin affected mining profits, generally. When price of bitcoin stands at high, mining profits for all looks much higher.
Of course, profits will be higher if price for BTC or ETH\LTC going higher
if the price of bitcoin is higher, the complexity of mining will increase. therefore, profitability will decrease.
It becouse BTC have constant number (21M) so any new mined BTC harder to mine than previous. And when last BTC will be mined... You have to mine something else... ETH for example..
Bitcoins will be mine for a very long time, we do not need to worry about the fact that we cannot mine Bitcoins.