Yes you're right if they switch off so fast (that is, within just a few periods) that difficulty is not adapted to their switching off. Then indeed, to get to the difficulty-lowering with a PC, I would still be stuck for a while with a HUGE difficulty as you point out. As I pointed out already, I was talking about evolutions much slower than the 2-week period of adaption.
We get here an extreme example of what I said, that during difficulty-decrease, somewhat less than 3600 coins are mined per day. Here, the step in difficulty, if taken at once, is so huge, that this "somewhat less" changes into essentially 0.
I'm assuming all changes slowly as compared to the 2-week adjustments.
I thought it illustrated quite well what I wanted to say. If difficulty goes down (for a long period), and you look what that means for someone who has been mining, it means that maybe some of them will mine more coins at same power, but at the expense of others who will mine less coins with less power, or some who will even stop mining. Not all miners who were mining before, can mine more coins at same power ! If some mine more, others have to mine less. If all of them do the same thing, then it simply means they all consume somewhat less (switching off part of their park) and will mine the same amount.
I was looking at the viewpoint of a random miner BEFORE difficulty went down, and how things happen to HIM.
That is like looking at a random soldier before he goes to war, and look at what happens to him. Some will die, other will become victorious. Just picking out those that will become victorious, and saying that going to war renders you victorious, was in my eyes similar as saying that when you're a random miner, when difficulty goes down, you'll mine more coins. No. Some will (like some soldiers will survive and be victorious). Others won't and will stop mining (like the soldiers who die).