The guy that hits 2000 in 10 averages 50 more hits a season. 10 years is nothing to scoff at. The average MLB career is like 6 seasons I think. To me that makes no sense. The only thing the guy that plays longer has is that he played longer and had more opportunity to amass numbers. That's the inherit problem with counting statistics.
But playing longer in and of itself is not something to overlook. If you play for ten years and put up numbers that would have gotten you into the HOF had you played longer, it doesn't get you into the HOF. Whatever the reason is for your not playing, the fact is you weren't around long enough. When a guy plays as a starter for 20 years, chances are he is one of the best of all time. You don't just do that by accident. So yes, they have a longer time to amass these stats, but you have to be good enough to have chance to amass these stats. And simply playing longer doesn't get you my vote. There are plenty of players who play 15-20 years and don't make the HOF. It is just PART of the reason you vote for someone.
EDIT: You said
So you'd vote for a guy who was never considered the best at his position, never the best of his generation, or decade, but that played for 20ish seasons and has high totals and claim they're one of the best of all time.
How can you be one of the best of all time without an era of dominance?
In certain situations, yes. For instance, Craig Biggio. If you play for 20 years and are putting up numbers like guys like Biggio did, even though he never was dominant, yes I'd vote for him. But the thing is, these players that play for 20 years and put up numbers worthy of getting my vote, typically would be at or near the top during their time period. Biggio averaged 174 hits per year over a 20 year career. That's not an incredibly impressive average...until you see that he did it for 20 years. That is one more than Chase Utley has averaged...but Biggio did it for double the length of time.
This is the small hall argument I've been making. If you start letting in guys who had great peak years, but the rest of their careers aren't as good (injuries, age, whatever the reason), you are going to have more guys every year getting into the HOF. There are plenty of players who have had some very impressive runs, but not the longevity.