if Horner were to be let go, who do you think would have a good chance for taking that seat?

I don't think Horner will be let go. To be honest, it sounds very bad, but in such a big org, when a top dog is bad, there will be more than 1 complaint.
But to playbthe game: Steiner and Binotto are no fit for RB. Will be someone with RB blood. Coulthard maybe. But I'd go for Vettel as principal. He knows racing.

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He has a great network, has racing knowledge, was a Red Bull junior driver plus 4 time world champion for Red Bull. Why not?
I know, it's out of the box. But Red Bull has a unique culture, they need someone who understands that.

never took him for the type
but you got some interesting points

I'm not sure he's the type. I just don't see Steiner as 'TP' for Red Bull. Red Bull is a cult, and he doesn't seem part of it.