Even when a degree wasn't required, usually the vast, overwhelming majority end up having one.

I'm one of the biggest critics of "Big Ed" and its generational multitrillion dollar scams. I'm in favor of it for airline pilots though, because its a barrier to entry that controls the supply and drives up the price of labor. Not quite AMA cartel style, but its something. We (pilots) don't really control that gateway though, so its not really up to us, other than the feedback we give shaping the interview process (which is actually pretty significant, as it should be).

I'm absolutely against the back room treachery of a secret "vault letter" though, and would love to see something like that challenged and eventually eliminated.

And your point that a "large percentage" of DL pilots haven't gone through the interview standard is still a false premise to base your argument on. The merger doesn't count, because mergers don't count for that ever. Besides NWA had a tough interview and selection process, and argueably one of the toughest paths for a new hire to the line of anywhere. But again, mergers don't count for your point.

The number of total pilots without a degree at DL is a very, very low percentage.