Moving on from a single pivot was done before Flick, because we didn't have a player to play the position; that's the whole point.
That player could be Zubi.
Rodri can play it and excelled at it for multiple seasons.
We definitely didn't use Rakitic as a double pivot; I don't know what you're talking about with that. He was a box-to-box midfielder in a 3-3.
We moved to 2 with busi because he was old and de jong can't play in midfield outside of a double pibot
I don't know what you mean by Pedri playing deep. His heat map for two seasons tells us exactly where he's playing, and that's as a CM, which is the exact same way he would play in a 4-3-3.
You were wrong on basically everything there
We played 442 most of the time under EV first year, with double pivot. This was Busquets best season between treble and retirement, same for Raki really.
Pedri plays as part of double pivot, which is niether a CM or CDM in the sense you are describing.
Pedri and Frenkie gets to play from deeper point in build up, with both doing it. This is the basic concept of double pivot. It helps both players in getting their best form and help the team escape pressing better.
You keep using the word "can" , and sure they "can" do it. But none plays his best there. As I said, even those who "can" play single pivot are better in double pivot. It serves no one any good to go back there.
And in his last season Xavi tried to force Romeu in single pivot, and later tried the same with Frenkie, and both failed miserably.