But Sampras is not a tier below Federer because the gap between 8 and 11 GS is not the same as the gap between 14 and 17. As you get higher up the gap shrinks even though the difference in number of GS is the same. As such, Sampras and Federer are on the same tier.
Fed has tons of stuff more than "just" 3 majors:
-masters, Pete is not even close
-WTF title
-weeks nr.1, consecutive weeks nr.1
-23 semis, Pete doesn't come close
-24 consecutive finals won, Pete doesn't come close
-more additional GS finals
-career slam
Also 3 majors in itself is enough to separate them. 3 majors is huge. That is career of Hewitt + Roddick. Also winning last 3 majors when you are past your best is a lot harder. Any additional major past 14 is insanely hard. That's why nobody in the open era is close to 17.
But even for the biggest skeptics who think 3 is not enough, Fed still has tons of stuff.
And I didn't even mention more than half of stuff Fed has over Pete including Olympic gold+silver.
Doesn't really matter, because Fed is a tier above Sampras. We can argue Fed had to deal with nr.2, five years younger and surface goat, otherwise he has 22 majors and CYGS. I think if we put Fed's career in context he has 22 majors, so we can argue that he is 2 tiers above Pete with 22 majors.
Agassi doesn't come close to Nadal as rival. Agassi was the best after Pete already won most. Also they are the same Age, while Nadal has age advantage over Fed.
People use some common sense. Fed had to deal with a rival who is arguably even better than Sampras himself, bad matchup and a lefty and has age advantage.
