Phreak actually said that stats disprove this. According to one of his patch breakdowns, toplane having an advantage very early is a good indicator of which team will win, but if that advantage develops midgame or later then it actually has little impact on overall winrate. Instead, any of the other roles getting a lead in the midgame or later is a better indicator for overall winrate than top. So, unless you are absolutely dominant in toplane from the get-go, you probably are not going to be a 1v9 machine anymore, and everyone else having a lead is going to be more important than you.
To clarify:
You're overstating the gulf in late-game somewhat. There are some top laners that still express their late game leads as game victory better than even the most snowbally bot laners (Kayle and Tryndamere are the biggest standouts). You're correct that the overall trend does shift toward bot lane as games go later.
My belief is that every role should have both solid early snowball and late game carry options. Darius and Kayle don't need to have the same agency curves as one another. That likely means reigning in some things as well as buffing others.