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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Great take. It’s in all of the organization’s best interest that he doesn’t get let go. Classic slow play is in motion. Once the season starts, does this really matter all that much apart from a few team principals trying to tap about it and enhance their own team’s position?

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.