There is no incentive for a mining hardware company to ship any of their hardware to pre-order customers after it is built, except maybe to avoid a lawsuit. 

1: They know their customers just spent all the money they had on their hardware.  So a lawsuit is going to be a far off, if ever, kind of thing.  Not that courts are fast enough to do anything about it anyway.

2: Now that the mining hardware company has a boat load of BTC income (mining) and regular old money (customer) they can drag any lawsuit out for a pretty long time at a lower cost than giving up the hardware, at least on the scale of how fast the courts move vs the difficulty.

3: If they get taken to court and lose/settle, they settle for an even refund with the customers (who are glad to just get their money back at all).  Since it was a pre-order there is shaky legal ground for any customers to get damages above refund.  Or even worse, the per-order people get their, now sadly, outdated "fully tested" hardware.


Here is how that conversation goes from the viewpoint of the mining hardware maker: "Why ship something on-time when we can "test" it for months, make a pile of BTC, and when we come up with the next step up, we ship out the "old" hardware that made us a boatload up front and during our months of "testing".  The suckers, err.. customers, won't have a leg to stand on legally if they come after us anyway, heck it'll be too late for them by the time they figure this out!".


Anyway, the difficulty ~could~ go down... but Moore's law, which might apply, suggests that will never happen with respect to hardware power in the future. 

It would take a concerted effort by all miners to keep the difficulty from going up.  If that were to happen, the guy/pool/whatever who breaks rank will be rewarded handsomely, possibly with a chance for a 51% attack.  No one else would want to see that happen, so they will all mine at full tilt no matter what.  Analogous to the concept of mutually assured destruction...