Longevity isn't the only argument.
I see four reasons why Sayers could be in and Davis not:
1. Sayers had almost no help on either side of the ball.
2. Sayers also was the greatest kick returner in NFL history.
3. The Bears' system didn't produced another great runner after Sayers left.
4. You shouldn't compare stats across different eras.
That's not to say that Davis doesn't belong. Just that Sayers has arguments in his favor that Davis doesn't.
I think the Priest Holmes comparison is infinitely more valid:
9 seasons, 109 games, 1734 att, 8035 yards, 4.6 avg, 86 rushing TDs, 334 recpts, 2945 yards, 8 receivng TDs
Depending on when you asked me, I would probably give Davis the edge over Holmes for having more dominant years. But again, the fact that the Denver system kept producing very productive RBs after Davis left hurts him.