"Thankfully this old rMBP I use is still running well (despite the screen covering deterioration in weird spotty ways)."

Does it look similar to this - http://www.staingate.org/ https://www.macrumors.com/2015/10/17/apple-mbp-ar-coating-qu... . My rMBP had the same problem and I got the screen replaced for free (outside warranty).