You are implying two things. One he is incompetent and two he will get a long term contract which will cost "a shitload of money to sack him". You might be wrong on both counts; he succeeds in steadying the ship and his contract is short term say to end of the season.
I'm not assuming anything special at all.
Will he sign a short term contract? Of course he won't, he's never done so before.
Will he bring in his own backroom staff? Yes, it's what he does every single time.
Will he sign a bunch of mercenaries in January? Yes, it's what he does every single time.
Will it be hard for us to get rid of those players? You bet it will.
So given all this, how is the post-Allardyce rebuilding supposed to go?