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.
By all means, dont spend a satoshi on mining hardware, because you do not understand the basics yet. The salary of miners is irrelevant (and many of them have been mining for years since bitcoins traded for $1 and so are independently wealthy now because of it). What hashing speed their machines have, is irrelevant. No one is going to turn off their asics unless operating cost (electricity) exceeds income, and that efficiency doesnt scale. We are very, very far from that with current Asics Difficulty decreasing, thats a joke. For the foreseeable future it will keep rising exponentially. If you think the current increase is steep, you havent seen anything yet, new generation asics are only just trickling in.
Mining hardware is priced per TH for the most part, buying more doesnt give you an advantage (in fact, it gives you a disadvantage. The more TH you buy, the more you compete with yourself). There is no advantage of scale in this game, except if you scale it to the level of building your own chips and miners, then suddenly everything changes. Well, even for that you are too late now and anything else is just a guaranteed waste of money.