By this logic, I shouldn't have made the top cuts I did with my non-Eel, non-Celebi deck. Also means my teammates playing Typhlosion and Truth wouldn't have made top cuts.
Just because it's widely played doesn't mean it's good.
To a certain degree, you have a point.
But the simple truth is the most played decks are the most played decks because they give you the best chance of winning. Let's look at the numbers, here are the first place finishes in Masters from States:
Zekrom/Eelektrik x24
Celebi/Mewtwo/Tornadus x18
Durant x4
Landorus/Terrakion x3
Donphan/Mewtwo x2
Straight Celebi/Mewtwo x2
Reshiphlosion x2
x1 Victories:
Straight Terrakion
Magnezone/Eelektrik
Terrakion/Tornadus
Magneboar
To a certain degree, you can chalk up Zekrom and Celebi having the most wins to being the most played. But even is you cut their win in half, they both have more wins individually than any other deck. Even if you chop them down to 1/4 of the wins they actually have, they STILL have more wins than every other deck. That's just too many wins to claim that they only win the most because they are the most played.
Now look at Top 8s:
Zekrom/Eelektrik x114
Celebi Mewtwo Tornadus x79
Durant x30
Landorus/Terrakion x13
Reshiram/Typhlosion x9
Terrakion x6
Donphan/Mewtwo x6
Meesie Mew x6
Terrakion/Tornadus/Mewtwo x6
Celebi/Tornadus x4
Magnezone/Eelektrik x3
Zekrom/Pachirisu/Tornadus/Shaymin/Mewtwo
Mangeboar x2
Truth x2
Six Corners x2
Mew/Unfezant/Vanniluxe x2
Electrode/Terrakion/Mewtwo/Reshiram x1Typhlosion/Magnezone x1
Terrakion/Tornadus x1
CMT/Vileplume x1
Chandy/Dodrio x1
Thundurus/Terrakion/Eelektrik x1
Cobalion/Kyurem/Electrode x1
Chandeulre (Fire) x1
Straight Donphan x1
Leavanny/Vileplume x1
Donphan and Dragons x1
Hippowdon/Landorus/Terrakion/Mewtwo x1
Yanmega/Terrakion x1
Reshiboar x1
Chandelure/Dodrio/Mewtwo x1
There is a LOT more variety here, but notice something? That variety disappears the deeper you get into the cut. Again, this is too obvious a pattern to chalk up ONLY to the sheer numbers of players playing the top 2 decks. There is a reason that Celebi and Zekrom win the most; they are the best. There is also a reason that Six Corners, Typhlosion, The Truth, and anything else with Vileplume DON'T win; they can't keep up with the top decks.