I agree to a certain extent. There are only 32 GM jobs in the NFL, 31 if you count that BB is GM at the patriots. This is a case of supply greatly eclipsing supply. Whether the candidates are good is irrelevant. If you don't jump at a change to be a GM at a certain point in your life, someone else might take it and potentially might do a great job. Of course, you don't go in blind. But at that level, every candidate who applies has belief that s/he is capable to doing a great job.
They are limited, but there will pretty much always be an opening or more every single year. And unlike young coaches, you don't seem to get too many more chances to be a GM if you blow it the first time.
I know there are some who would argue you take the first job available and run with it and hope for the best, because you may never get that chance again. But Josh and Matt and Nick have all had requests before, and they've interviewed or passed on interviews, and they're still getting chances. There will be other opportunities in the future.
And even if there's a bad year, there are chances to do good again. Josh got ran out of town in Denver, went to the Rams, didn't do much, but came back and did well with the Patriots and got his chances. BB the same after Cleveland. Kyle Shanahan was with the Texans, had a good start in Washginton but eventually bombed there, didn't do much in Cleveland, then lights it up in Atlanta and starts getting HC interviews.
Arians and Zimmer didn't get HC jobs until their 60s. Del Rio did well for a while in Jacksonville, got fired, bounced around a bit as a coordinator before getting another opportunity. At the same time, there's only so many chances to be the head guy before you're branded Wade Phillips or Norv Turner types, great coordinators who can't handle the HC job. I think both would do better now just from experience, but it's doubtful they get that chance.