the miners may feel worry about the exponential increase in difficulty. That’s true as long ethereum mining is very lucrative these days, and the miner can earn about 30 % return of the invested money which is very crazy, so you can see the competition in buying the graphic cards and shortage of their supply, besides the newcomers’ who urgently demand the graphic cards. All these means a lot of hashes are introducing and normally the difficulty increases.
According to the chart the difficulty has doubled within 30 days from about 450T to 900 T.
According to this rate of difficulty, the hash rate of your graphic card will be decreased by the end of the next 6 months about 25%. Which mean if your card hash rate now is 20 MH/s then after 6 months it would be 15 MH/s. but don’t worry also according to the rate of increase in the ethereum value, you still earn the same amount of money in dollars ($$) or even more after 6 months.
one last thing is the cryptocurrency world is very active and every few days a new born of digital coin comes to life, so the chance is always there for the miner to dig in with more profits.