Marino went from a rating of 80 at 37 years old to a 67.4 at 38. His yardage at 38 was the third lowest in his career, the other two being 1993, when he hurt his Achilles, and his rookie year. My memory is that he was in slow decline, but the numbers say other wise. his final 4-5 seasons were consistent Marino-esque. The last one is the only one where I would say that might qualify as a steep slope, but not necessarily a cliff. Favre at 41 had the lowest yardage of his career at 2500 something, but he only started 13 games. His yards per game were also off, the only time he was under 200 yards per game his entire career. Again, steep slope, not a cliff.
I think you're right on the money with your analysis. If you dig further, some of the elite QB's excelled late in their career. Tarkenton lead the league in yardage his final season, the only time he did it. Elway's numbers in 12 starts his final season were consistent with his average throughout his career.