Ever since 3DS, Diddy Kong, Sonic, and now Luigi a tiny bit are the only top tiers who have been nerfed, and deservedly so, so I don't know why people keep saying "don't nerf the high tiers, buff everyone else!" The top tiers are outliers who had strengths that far outweighed everyone else. Diddy's absurd KO power off of easy setups, Rosalina's original inability to be grabbed and everything else about her, Sheik completely dominating the neutral and basically being at no disadvantage ever at any point. Characters need to be balanced around high tier: Strong and cohesive movesets and attributes but still with distinct disadvantages. Not top tier, who are basically great in every category except for the ones that they are absurd in. Sheik still needs to be nerfed, and then we'll see. I don't think anyone can move into that spot, even if Rosalina will probably be the undisputed strongest character.
And to address the "someone else will become top tier" argument, there's always going to be a top tier because that's just how lists work. The goal is to make it so that there are compelling reasons to choose characters other than that top tier. The problem is when characters who do everything better than everyone else (Sheik) are the top tier.