If you want to start trading only with bitcoin and fiat then it will not be constant all the time as sometimes the price come to stability and sometimes a drop down occurs in the price so you have to trade with a number of coins and it is seen normally when one coin start to decrease then any other at that time increase in price so as a trader you will start getting profit from that second coin and with that your profit will be fixed for you.
Sure, profits in trading cant be constant because of the coin's volatility. Just like every other trade, fixing the income will help business to avoid extravagance and excesses can be conserved or reinvest.
Exactly, in trading we cannot say your profit is stable or even fixed since the value of coin really changes every now and then, which excites every traders because of the volatillity of coins, and avoid extravagance of every trading industry .
But as I said that if the person will start to trade with different coins then he will be able to get a steady income from his trade as we often see that when one coin go down in price then any other coin come and increase in value so he will have the chance to get the profit.