I am not a native english speaker so maybe I didnt express myself well
I believe that most of the people that are mining (like 70 to 90% of them) are people with average or mid grade salary (in real life jobs) and they have average to low grade hardware (most of them usb sticks,5 GH/s machines and some of them 500GH/s machines tops...)
So for those people if the difficulty exceeds lets say 6 or 8 billion they sure will quit since their hardware wouldnt even get them a thusandth fraction of a bitcoin per day in that difficulty so if and when they quit the difficulty should drop down (since 70% or 90% of the miners dont mine anymore having as a result the average time for a block to be found to increase noticably)
So at that time it should be a good idea do get into mining with a strong piece of hardware thats what I was talking about mine for a few weeks and then when people keep on coming back you just wait for the difficulty to raise up again.
Your English is just fine and certainly better than some so called native speakers can manage, when I talk about misapprehension I mean that you seem to think any future hardware will somehow be different, or have a different effect, to the new hardware being produced now. New hardware comes out, everyone has the same idea, the difficulty rises, everything settles back into the status quo at a much higher difficulty and chasing fewer and fewer coins as the reward is halved each four years or so. The more you "invest" the more you need to recover.
What may happen is that smaller players are squeezed out of mining process at some point by the running costs and then the hashing power becomes concentrated in fewer hands but that itself is self defeating for the bitcoin concept, if people don't have btc they can't use them and if they aren't using btc then it really isn't worth anything much. It has no intrinsic value, the only value is that imputed by the people who use it to trade with.
In the very, very unlikely scenario that difficulty drops, those who had been forced to switch off will simply switch back on again because the difficulty has dropped and mining became viable again for them. It's also the reason many won't switch off in the first place and continue to pour real wealth into mining, basically speculating that one day they can recoup that investment with the btc generated. It's a huge gamble and one that people really ought to be making with their eyes wide open.
I'm not negative towards bitcoin at all but a lot of the talk around mining btc and making a "profit" is borderline psychotic imho.