Complainers are going to make it impossible for manufacturers to produce new equipment. Sure, it would be nice if everything was "perfect," but nothing ever is. Even the D600 oil spot "issue" was turned into a ridiculous mess for no real reason. (I'd much rather have it over lubricated and slinging a little oil during "break in," as opposed to under lubricated and me not knowing about it.)
Why are you trying to trivialise a fault Nikon themselves have admitted, and is something they are looking into?
I'd much rather the camera was built and designed properly than to live with a problem, especially one that could lead to me having to potentially do a wet clean each time I used the camera.
It's not a "fault," it's a "characteristic;" just like the D8xx's lack of an AA filter causing moire is. If that situation is problematic/common for you then pick a different camera. It's not even unique to the D750, so you probably need to pick something without a PDAF module in the mirror box.
The decision to remove the AA filter was a design choice for sharper images out of the camera, and I think bringing both the D800 with and without the AA filter emphasized that people had to make a choice, and what the consequences were with that choice. Making a design mistake is totally different to a fault. In various recent Nikon cameras there have been faults, left AF points not working, liquid getting splattered over the sensor, LCD not showing correct colours, white spots in long exposures etc. There are probably some I've missed. Nowhere did I see people getting prior warning of these 'characteristics', as they did when Nikon started releasing cameras with the AA filter removed. Nikon has repaired, replaced, and sometimes re-designed existing models, and even brought out a whole new camera for some or all of the 'characteristics'. Should users should just have kept quiet and lived with the problems?
Can they "fix it?" Sure, with a redesign... a taller body to allow a taller shield, put the AF mirror on a separate mechanism that drops over the AF module, make the entire mirror assembly flip down instead of up, move the AF module into the pentaprism area, eliminate PDAF, etc. etc. And maybe they will do something like that in the future; but I don't see a "fix" for any of the cameras that currently exhibit this characteristic. *Maybe if* the current shield is shorter than it could be they'll be able to modify it (but what a mess that would turn out to be).
Surely if they have to fix a problem with a re-design, then the thing wasn't designed correctly in the first place.
If Nikon are saying that they are looking into a problem, then they consider it a real problem. For most users faults are not 'characteristics'.
Most users may indeed be very happy and have no problems, but, there are some that are not. Of course you hear the people with problems more, but then if I spend money on a product and it is faulty, then I would want to make it known, and ascertain whether I was alone and unlucky, or part of a larger problem. The more expensive the product, the more upset people may be, as they may have saved/got into debt, to get said item. If people don't highlight problems, manufacturers may not find out about them, (unless they already know) and may do nothing about them.
The bigger any problems, which you will not find out without people communicating with each other, the more likely the manufacturer will do something about it. Small problems they can ignore and brush under the carpet.
Nikon are not the only manufacturers bringing cameras out with problems, but they seem to be doing it more than most. For all of them, is it poor design or poor quality control? Depends on the problem. But never just accept it.
And because people discussed and raised the issue, Nikon will actually do something about cameras with the problem. If people just accepted it as a 'characteristic' that would go away, eventually, possibly, I doubt even less would be willing to get a D600.