I see zero reason why Allen is a safer bet than Lamar to continue being an effective QB once he starts to lose his mobility.
To the extent that metrics can help with this, consider these for 2022:
- From PFR:
- Passer rating: Allen 96.6 (#8), Jackson 91.1 (#15T)
- QBR: Allen 71.4 (#2), Jackson 59.0 (#9)
- Completion Percentage: Allen 63.3% (#22), Jackson 62.3% (#24)
- TD Percentage: Allen 6.2% (#3), Jackson 5.2% (#9)
- Interception Percentage: Jackson 2.1% (#15), Allen 2.5% (#24)
- Yards Per Attempt (YPA): Allen 7.6 (#8), Jackson 6.9 (#21)
- Net Yards Per Pass Attempt (NY/A): Allen 6.87 (#6), Jackson 6.05 (#19)
- Adjusted Net Yards per Pass Attempt (ANY/A): Allen 6.99 (#6), Jackson 6.12 (#15)
- Completed Air Yards per Completion (CAY/Cmp): Allen 7.5 (#2), Jackson 6.3 (#13)
- Completed Air Yards per Pass Attempt (CAY/PA): Allen 4.7 (#2), Jackson 3.9 (#14)
- On Target Percentage: Jackson 75.4% (#19), Allen 73.7% (#23)
- Bad Throw Percentage: Allen 16.6% (#20), Jackson 18.1% (#24)
- Sack percentage: Allen 5.5% (#10), Jackson 7.4% (#21)
- From PFF:
- Passing grade, minimum 200 dropbacks: Allen 85.8 (#3), Jackson 72.3 (#16)
- Deep passing grade, minimum 20 deep pass attempts (20+ yards beyond LOS): Allen 92.7 (#6), Jackson 62.7 (#31)
- Passing grade under pressure, minimum 50 dropbacks: Allen 72.9 (#2), Jackson 44.0 (#30)
- Passing grade kept clean, minimum 50 dropbacks: Allen 87.6 (#7), Jackson 85.3 (#11)
- Percentage of Pressures Charged to QB, minimum 50 dropbacks: Allen 18.8% (#28), Jackson 33.9% (#36T)
- Big Time Throw Percentage: Allen 7.6% (#1), Jackson 5.0% (#4T)
- Turnover Worthy Play Percentage: Jackson 2.3% (#5T), Allen 4.2% (#29T)
- Adjusted Completion Percentage: Jackson 74.6% (#20T), Allen 74,2% (#23T)
- From Football Outsiders:
- Passing Value Over Average (VOA), minimum 200 pass attempts: Allen 15.7% (#5), Jackson 3.8% (#16)
- Passing Defense-adjusted Value Over Average (DVOA), minimum 200 pass attempts: Allen 15.9% (#5), Jackson 5.5% (#14)
- From rbsdm:
- Expected Points Added (EPA)/play: Allen 0.220 (#4), Jackson 0.109 (#12)
- Adjusted EPA/play: Allen 0.231 (#4), Jackson 0.117 (#12)
- Success rate: Allen 53.0% (#2), Jackson 49.3% (#14)
- Completion Percentage Over Expected (CPOE): Allen 0.9% (#11), Jackson -0.6% (#22)
- From Next Gen Stats:
- Completion Percentage Above Expectation: Allen -0.9% (#20), Jackson -2.4% (#29)
That is 28 metrics, though there is plenty of overlap between the metrics from different sources, so not truly measuring 28 different things. All are rate/efficiency metrics to avoid the issue that Allen played more games. Jackson was ahead of Allen in just 4 of them.
There are plenty of differences that make it challenging to compare two players like Allen and Jackson, since their coaching, schedule, teammates, weather/field conditions, etc. are different. But using these metrics as a proxy, Allen appears to be a much better passer than Jackson based on the 2022 season, which is the most recent evidence we have.