Then what?
Europe won't renegotiate, so the very policy that he won the Tory leadership will have immediately failed.
The public failure of your prime policy. The policy that you destroyed the previous government in order to pursue.
When your prime policy fails - as Johnson's is bound to - then you have to resign.
Elected leader on Friday and have to resign on Monday.
Cameron promised a new treaty, failed, Johnson forced him out.
May got a Brexit deal, Johnson voted it down and forced her out.
Then Johnson becomes prime minister promising a new deal . . . ?
If he doesn't it'll be a general election, and he'll lose that.