When Sampras won his 13th and 14th majors, I don't think Sampras thought that any player would get near those totals for decades, especially as the 1998-2003 period had a load of different major winners.
From 1998-2003 (a 6 year period), the following players won majors:
Juan Carlos Ferrero
Yet from 2004-2012 (a 9 year period):
Juan Martin del Potro