Not sure why people blindly believe this but I never mained Fox lol. I've been a Pikachu main since day 1.
On paper you would think that if I put more time into a high tier, I would be a better player. But in practice it didn't turn out that way.
For the first ~10 years of my career, I spent much more time playing Falco than any other character. But when I entered tournaments, he couldn't perform to the heights of my Pikachu. I spent an absurd amount of time grinding out Fox as well, but for whatever reason my Pikachu still performed better even though I didn't play him as much.
Eventually I leaned into playing Pikachu a lot more and I think he's the character I have the highest potential with. It might sound crazy because he's a lower tier, but sometimes you have to be the one to write the book if it's something you believe in. Years ago I sort of carried him from near bottom tier to high-mid tier, and I think he can go higher thanks to the other amazing Pikachu players that exist now.
There is definitely still a use for other characters as secondaries though. I don't think all of the time I put into practicing them was meaningless. They help in certain matchups, whether against certain characters, stages, or specific players.
But to those saying I would be a better player if I just stuck to a high tier the whole time, I personally think that's wrong even though it might seem right on paper. I think certain people just have a knack for obscure characters and different mechanics that the current top tiers don't provide, and I happen to be one of those people.