As Keir said in PMQs today, it is only a matter of time now whether the prime minister is rejected by the public, the Conservative party or does the decent thing and resigns himself, and those are still the three ways forward.
If you’re asking why Keir Starmer used the language he did today, it was very simply because we have now got, after all of the various tortured attempts at explaining what happened, an on-the-record acknowledgement in parliament from the prime minister that the rules were broken, that there was inappropriate behaviour in Downing Street, that he was involved in it, and, by any reasonable measure, I think that meets the test of calling on him to resign.