The problem the Tories have is that a lot of people only voted for them last time round because of "good old Boris". Some of them are finally seeing him for what he really is but many would still vote for him if he broke into their house and shat on their kitchen table.
Replacing him must seem like the right thing to do right now if you're a Tory backbencher, but they'll still be wondering if he's unbelievably more of an asset than a liability.
Next week's by-election could change all that.