The Tories have probably guessed that Johnson will destroy the Tory voter base because the public sees him as toxic. Something else could have happened behind the scenes.

I have long suspected he was only needed as the face of Brexit. Similar to the Trump-to-Biden move in the US, the establishment now needs a ‘sensible’ face. I think Johnson was ‘loved’, in a way. Not by the majority of normal island, but by the vocal minority that gets air time and front pages. He is the UKIP/BNP/EDL’s Tory. He was able to go far enough right to unite the right on Brexit. That’s still a minority bloc, but in electoral terms, it’s enough to secure a Parliamentary majority with the Tory loyalist voters (monarchists, reminiscent for the Empire, hard-work-will-free-you, etc) and the swing voters who rejected Corbyn (but would have voted for e.g. Blair or even Miliband). These voters may not have liked Johnson, but they could overlook his character flaws. The papers ran with the one-sided, the public love him angle.

Unfortunately, the machinery is now moving into the next phase, which includes selling off Britain’s remaining nationalised industries. The coming alternative is a better dressed and more carefully spoken neoliberal, from either within the Tory party (Sunak, maybe) or from Labour if there’s an election (likely Burnham, depending on the timing). Imagine if someone who appeared competent demolished the NHS? Centrists would lap it up like they did with Cameron (austerity), Blair (mixed economy), and Thatcher (deregulation). The narrative is already there. People have long been talking about the need to introduce fees to use NHS services (beside national insurance contributions), but nobody would believe it if it came from Johnson.

In short, I think this development started when it became clear that Johnson could secure Brexit (tick) but would not e.g. achieve privatising the NHS.

Edit: spelling