Frank Gore's career rushing total. Great back, don't get me wrong, top 20 and a hall-of-famer. But he was never an Emmitt Smith/ Walter Payton / Jim Brown / Barry Sanders type back that you had to gear your whole defensive game plan around, though his career rushing stats would put him in that crowd (and toward the TOP of that crowd).
Better example: Lenny Wilkins as an NBA coach. 2487 wins! Winning percentage 53.6%. Playoff winning percentage 44.9%. 32 seasons, 1 NBA championship, a 52-30 Seattle team.
Lenny Wilkens | Basketball-Reference.com
Is Gore a top 20 all time RB? His longevity has been a sight to behold, but what do his stats say? He exploded for 1600+ yards and 61 receptions in 2006. From 2007 on he only topped 1200 yards twice, his YPC only exceeded 4.5 three times, he only scored 10 TD once, his receiving production has been decent, but never approached his 2006 numbers.
Many of the most dominant players in league history had very short bursts of dominance: Gayle Sayers, Megatron, Patrick Willis, Gronk.
If you define greatness in terms of individual dominance over a short period of time, rather than the amassing of numbers, you reveal a list that runs dozens of names long. Priest Holmes, Tiki Barber, Sean Alexander, Marshawn Lynch, Corey Dillon, Jamaal Charles, LeSean McCoy, Fred Taylor, Ricky WIlliams, Clinton Portis, Matt Forte were all superior runners in their primes, and none among that list show up on anyone's top 20.
I'd compare Gore's case to that of Bert Blylevin, a never-dominant player who happened to be pretty good for so long he accidentally became known as great.