Bitcoin mining is a costly business. If you do not have enough capital, it will not be profitable. Nowadays miners are carrying out their expenses by minimizing their costs.
But difficulty of the mining business does not adversely affect the use of bitcoin. If somebody leave mining, the level of difficulty decreases. Then mining would be less costly.
You are totally right there, if someone leave mining bitcoin percentage of mining will rise. But i don't think that someone that invested a lot of money, and for mining to be pay off you need really good computer, they won't leave mining bitcoin so easily. I have heard that mining in this days are too hard, and it doesn't pay off. I was thinking to mine, but when i find out what you need, i don't have that much money...
Anyone who starts to mining will not leave mining unless it is a big mistake.
Because the price of bitcoin is constantly increasing. Even if btc is not profitable today, it will be profitable for future btc value.