Federer lost like 20 matches in 2004-2007, his reign was an actual reign, not just being "barely" better than anyone else from his generation like Sampras did. Heck, guys like Moya, Rios, Muster, Rafter could take away the top spot from Sampras while Federer lost his top ranking in August 2008 to a GOATing Nadal.
Today's tour is pretty much one surface, all or nothing, homogeneous
environment. We pretty much always have at least 3 out of 4 top seeds at
semi final of all slams.
Today, #1 dominates the entire tour, all power baseline game. Either you #1 for
all year or your are #2. Other smaller factors are 1. 32 are seeded. 2. there
is no bonus point if lower ranker beats higher ranked players and so on.
All these promotes very few top players dominate the tour all year long
so that fans see familiar players on TV on weekends.
IMHO, the tennis of 70's - 90's were unique period in tennis history. We had
true varieties of players and conditions that brought out all aspect of tennis
in the way tennis meant to be played.