In 1989, Denver had the #1 defense by points allowed. What would you expect a team with the #1 defense, and the best QB of all time to do in the Super Bowl? Probably something other than lose 55-10 while the QB goes 10/26 for 106 yards and 2 INTs. Elway was the weak link on that team, not the reason they got to the Super Bowl.
Plummer now holds the single-season Denver yardage and TD records, and has more fourth-quarter comebacks per game played than Elway. (Fourth-quarter comebacks is a BS stat which measures opportunity, but people talk about it all the time in Elway's context).
Yeah, and that number one defense got trampled in the Super Bowl for 55 points. They fell behind quickly. You try putting up good numbers when you are already down 27-3 at the half.
Here's Denver's drives of the first half that game

23 1–10 Elway pass to M. Jackson right underthrown, incomplete.
D 23 2–10 Elway pass to M. Jackson middle overthrown, incomplete.
D 23 3–10 Elway 2 run evading pass rush (Fagan).
D 25 4–8 Horan 41 punt, Taylor fair catch.
D 26 1–10 Elway 27 shovel pass to Humphrey (Lott).
SF 47 1–10 Elway shovel pass to Humphrey incomplete.
SF 47 2–10 Humphrey 10 run middle (C. Brooks).
SF 37 1–10 Humphrey 6 run right end (Walter).
SF 31 2–4 Humphrey 3 draw middle (Millen).
SF 28 3–1 Elway 3 run middle (Millen).
SF 25 1–10 Elway shovel pass knocked away (Haley), incomplete.
SF 25 2–10 Elway pass to Mobley left overthrown, incomplete.
SF 25 3–10 Elway pass to M. Jackson right incomplete.
SF 25 4–10 Treadwell, 42–yard field goal (6:47).
D 49 1–10 Humphrey 1 run left tackle (Fagan), fumbled, C. Brooks recovered for San Francisco at SF 46.
D 25 1–10 Elway pass to M. Jackson broken up (Griffin), incomplete.
D 25 2–10 Elway pass to Kay broken up (Lott), incomplete.
D 25 3–10 Elway 7 screen pass to Winder middle (Holt).
D 32 4–3 Horan 37 punt, Taylor fair catch.
D 21 1–10 Humphrey run left, loss of 3 (Millen).
D 18 2–13 Humphrey 2 draw middle (Holt).
D 20 3–11 Elway pass to M. Jackson middle dropped, incomplete.
D 20 4–11 Horan 30 punt out of bounds at 50.
D 16 1–10 Elway 4 pass to Humphrey right (Kugler).
D 20 2–6 Elway 6 pass to Kay middle (Millen).
D 26 1–10 Elway 8 pass to Johnson middle (Kugler). Two–Minute Warning.
D 34 2–2 Elway pass to Johnson right incomplete.
D 34 3–2 Elway pass to M. Jackson right broken up (McKyer), incomplete.
D 34 4–2 Horan 42 punt, Taylor 17 return (Dennison).
D 37 1–10 Elway pass to M. Jackson deep broken up (Wright), incomplete.
D 37 2–10 Elway 12 pass to Sewell middle (Romanowski). Denver–first time out

12).
D 49 1–10 Elway pass to M. Young left sideline incomplete.
D 49 2–10 Elway pass to M. Young right sideline incomplete.
You don't think Elway being 6-19 for 64 yards in the first half might have something to do with the fact that they were down by 24 at that point? Or for that matter, the fact that they were playing against the
actual #1 QB of the era?
What about two years earlier, where Denver was up 10-0 and Washington scored 42 unanswered points? I suppose that was all the defense's fault and had nothing to do with Elway's poor performance.
Or the year before that, when the game was iced when Elway threw an INT deep in his own end while down 26-10.
Fact is, the AFC was horrendously weak in the 1980s. Starting with San Francisco's win over Cincinatti in 1981, the NFC went 15-1 against the AFC in the Super Bowl over the next 16 years. If Denver were in the NFC, they wouldn't have gone to the Super Bowl in any of those years. In 1987 there was no AFC team with 11 wins; in 1989 the Broncos were the only AFC team with more than nine. So I really don't put much credence in Elway's AFC playoff stats as an indication of success.