Here's something else. It's pretty rare to get solid, consistent production for an extended period of time from any group. Here are the number of productive (non-zero VBD) seasons and how many players achieved each of them.
Top 10 RBs (there are actually 21, not 22. I miscounted earlier)
10 - 2 (Sanders, Faulk)
9 - 1 (Bettis)
7 - 2 (James, Taylor)
6 - 1 (T. Jones)
5 - 3 (R. Williams, J. Lewis, Hearst)
4 - 1 (Byars)
1 - 5 (Enis, S. Smith, Fullwood, Bo Jackson, Thomas)
0 - 6 (Highsmith, Carter, Phillips, Biakabutuka, Worley, Vardell)
AVG: 3.48 productive seasons
Other 1st round RBs (not including top 10 RBs) (also a miscount - there are 42, not 41)
12 - 1 (Emmitt Smith)
8 - 2 (George, Dunn)
6 - 1 (N. Anderson)
5 - 3 (Alexander, R. Hampton, J. Stewart)
4 - 3 (Metcalf, A. Smith, L. Russell)
3 - 1 (R. Smith)
2 - 9 (L. White, Gary, H. Williams, Humphrey, Heyward, Wheatley, Bernstine, Stephens, Harmon)
1 - 7 (R. Edwards, Kaufman, L. Hampton, Salaam, Muster, G. Green, Palmer)
0 - 15 (Dozier, D. Thompson, Carter, Adams, Hill, Avery, Dupard, Vick, Dayne, Broussard, Sewell, Flagler, T. Smith, Canidate, Dunbar)
AVG: 2.12 productive seasons
1st round WRs (miscounted earlier. 53, not 54)
16 - 1 (Rice)
12 - 1 (Moss)
11 - 1 (T. Brown)
10 - 1 (Harrison)
9 - 1 (K. Johnson)
8 - 2 (Holt, Galloway)
7 - 2 (H. Moore, R. Moore)
6 - 6 (Sharpe, Irvin, Rison, Miller, Burress, Glenn)
5 - 4 (Moulds, E. Brown, Jeffires, Kennison)
4 - 3 (Conway, Toon, Morton)
3 - 3 (Alexander, Boston, McDuffie)
2 - 5 (Pritchard, Harper, J. Hester, Sherrard, C. Johnson)
1 - 10 (McGee, W. Davis, Dawkins, Hilliard, Taylor, Hill, Stokes, Ingram, Warrick, Westbrook)
0 - 13 (Cox, Dyson, Nash, Soward, Carruth, Anthony, Nattiel, Collins, Morris, Lewis, Edwards, Green, Howard)
AVG: 3.49 productive seasons
And, for more fun, here are the productive seasons divided by average career length:
Top 10 RBs: 3.48/8.24 = .422
Other 1st round RBs: 2.17/6.9 = .307
1st round WRs: 3.49/8.6 = .406
So even if their careers last for a fair amount of time, none of these groups is productive for even half the time that they are on a roster in the NFL.