Maybe there are a couple buried somewhere outside Nuremberg. 
OK.
Second, discussions of this issue aren't coming from the Left. They are coming mostly from non-political whistleblowers in the military and Republicans in Congress.
I understand the Left isn't exclusive in this discussion.
Third, the UFO issue has been stigmatized as "silly" at best and "crazy" at worst such that anyone reporting UFO in the military has lost their position (particularly pilots), credentials, and most certainly any security clearances that they might have had.
I was not aware of that. So you are saying those F-18 pilots a few years ago that sighted and pursued one, or more UFOs off the Coast of Southern California were subsequently sacked? I did see the tracking video. It bore all the hallmarks of UFO/Bigfoot video by being blurry and indistinct.
Fourth, the media has supported the military in stigmatizing the issue and marginalized anyone who tried to report or write about this issue (outside of fringe sci-fi or hobbyist publications).
Well, the planet is a big place. Is there some good reason these alien craft never seem to crash anywhere other than inaccessible regions of the United States accessible exclusively by covert US Government elements?
What went wrong with alien quality control? Why did these alien craft only start crashing in the past 70 years?
Why haven't these alien craft crashed elsewhere throughout history and at least some of the crash sites preserved by locals as shrines?
Why have no crash sites been discovered by modern relic hunters scouring remotest regions of the globe to retrieve and restore crashed warbirds?
?
“Why did these alien craft only start crashing in the past 70 years?”
Good question.
One explanation is that modern radar and other military technology can affect the craft in various ways. That is why many of the crashes are near military bases and not just in random locations.
So you are saying those F-18 pilots a few years ago that sighted and pursued one, or more UFOs off the Coast of Southern California were subsequently sacked?
The pilots who sighted and pursued the tic-tacs off San Diego (and the radarmen involved) were all separated from the Navy or retired when the videos were leaked to Chris Mellon, a former Deputy Assistant Secretary of Defense for Intelligence. Mellon took the videos to some very solid reporters (Leslie Kean and Ralph Blumenthal) who wrote the story and got it published in the NY Times (to the Time's regret).
The videos leaked are the worst of the bunch according to members of Congressional intel committees who have now seen the classified versions (classified because of the sensors/cameras that recorded them on F-18's and the USS Nimitz & Princeton).
The pilots and radarmen only went public after the Pentagon confessed the videos were real and indicated that they would not take action against anyone involved including pilots and radarmen.
Regarding crashed vehicles, I'll let someone else answer that question. I think you raise valid points but I don't have the answers.
As an aside, though whistleblowers recently have been talking about recovered vehicles, this hasn't been a major part of the discussion until recently and very little is known about it. Ergo, I'm not sure quite what to think of this matter.
Because, of course, a propulsion system that distorts gravity and space time would lend itself to perfect photography.
What went wrong with alien quality control? Why did these alien craft only start crashing in the past 70 years?
Who says? According to the whistleblower, some of the craft recovered were done so on archeological sites. There is reason to believe that in several instances people/companies/academics have stumbled upon, let's say, interesting things that have been swept away by government agencies, never to be heard from again.
Our government, and the governments of several other countries, have been actively seeking out such sites for at least the past 80 years and some would say the past 125 or so. There is good reason to believe that the Smithsonian has been collecting...things...since it's inception in cooperation with the US government for it's own reasons. Just because YOU haven't heard about it, doesn't mean it doesn't exist. There are a plethora of findings, artifacts, fossils, and other nifty stuff that, back in the 19th century, "they" decided we couldn't handle.
This mindset just continued. It became a case of "we can't tell them now, because we misled them before." Year upon year. Decade upon decade. They got so used to lying to us it became institutionalized and they got very good at it. When the pesky FOIA came along, they just privatized most of operations with deniable government oversight.
If you get a chance, check out what happened in Varginha, Brazil in 1996. This is a very well documented UFO crash with many of the people involved still alive and now willing to talk about it. Within hours of the crash, planes from the USAF were landing in Brazil and taking over recovery. Hours.
But, like I said in an early post, if you have made up your mind to not believe, no amount of evidence will change your mind.
Maybe there are a couple buried somewhere outside Nuremberg.