It’s definitely true that Kyle Shanahan's system elevates quarterbacks, and Dak is absolutely a more physically talented passer than Jimmy Garoppolo ever was
But that actually highlights the exact team-building problem. San Francisco was able to build that elite, star-studded roster with a top-tier run game and defense precisely because Jimmy Garoppolo and Brock Purdy were playing on cheap, team-friendly contracts. Because the 49ers weren’t paying their quarterback market-setting money, they had the leftover cash to buy Christian McCaffrey, Trent Williams, Nick Bosa, and Fred Warner.
To answer that question directly: No team has ever won a Super Bowl with a 32nd-ranked defense because a 32nd-ranked defense means you are the literal worst in the NFL.
But that scenario doesn't apply to Dallas. The Cowboys have never had a 32nd-ranked scoring defense during Dak's career. In fact, during his playoff runs, the defense has usually been a strength. In the 2021 playoff loss to the 49ers, Dallas had the 7th-ranked scoring defense. In the 2022 loss to the 49ers, they had the 5th-ranked scoring defense. Even in the 2023 season collapse against Green Bay, the defense finished the regular season ranked 5th in points allowed. They had a bad game against the Packers, but the unit itself was statistically elite.
Ultimately, it sounds like we actually agree on the biggest point here. If we both agree that Dak isn't an elite quarterback and shouldn't be mentioned in the same breath as guys who carry their teams to championships, then the debate is pretty much settled. At his contract price, the expectation is elite performance, and if he isn't giving them that in January, the team building math just doesn't work out.