Anyone thinking difficulty will only go up 400% or 1000% and flatline is likely going to be poorer for that faulty logic.
And why is that so hard to imagine? The madness is tied to the price of bitcoin, so it will stop at some point unless the value of the bitcoin goes up.
1) Because the amount of EXISTING pre-orders are is already more than that.
2) The margins on ASICs are still very high so once sales slow down to a trickle because annual ROI drops to something pathetic like 20% per year, the producers can cut the price by 50% and sell even more, then cut the price by another 50% and sell even more and by then have the funds to finance smaller/faster/better/cheaper ASICs.