I think both quality and longevity matter in equal measure. Just as you won't get in with a mediocre 20 years you also won't get in with one single great year. The question is where you think the line is.
Aside from the 2 super bowl wins, is there a real difference between Terrell Davis and Shaun Alexander?
And let's be real, Davis really had 3 seasons of greatness, and one good one. And it's not like the rest were average or mediocre, they were straight garbage (doesn't really matter why). He just doesn't belong. There isn't enough there.
I'm on the fence with Terrell Davis. I agree that he had 3 great seasons, but only one of them were all-time. If we're gonna induct a player for just 3 seasons, all three should be all-time. Hell, Priest Holmes and Tiki Barber put up same/more all-time seasons and they get no consideration. Furthermore, Clinton Portis, Reuben Droughns and Mike Anderson put up productive seasons after TD's injury.
On the other hand, it's not just 2 Super Bowl games, as you put it, that distinguishes Terrell Davis. He didn't just show up in 2 games, but his entire 8-game playoff career is exceptional. 8 games = 1,140 yards (5.6 YPC), 12 TDs.
Overall, I'm not a fan of putting in a 3-great-season-RB? However, if you're going to put in one, Terrell Davis is the one to put in.