I have two observations:
1) Cameron broke the mould, which Johnson & Truss have followed: they all wanted to be Prime Minister for the sake of being Prime Minister rather than because they had their own beliefs and policies. Compare with, say, Thatcher & Blair who both had a vision for the country and that was their motivation.
When he was first elected PM (2010?) I was working with someone who had been at Oxford with Cameron (and still kept in contact - she went to his 50th birthday party at Chequers) She always said he “was a nice bloke, but doesn’t really have any convictions. He just wants to be top of the pile“ and this was bourn out through his premiership. Johnson famously wrote two pieces, one for staying in the EU, then other for leaving, and only settled on the latter as he thought (correctly) it would be his route to power (although I don’t think he expected, or wanted, Leave to win). And now we (are about to) have Truss - who’s political journey has gone from Lib Dem, to Remainer, to ardent Brexiteer to further her own career. In between we had May who at least did the job out of a sense of duty. I don’t agree with much of what she did, but at least she did it as she thought it was the right thing for the country. I often wonder how different things might have been had she been at the helm during Covid. (I certainly don’t think we would have had partygate or excused staffers for travelling to Durham to check their eyesight)
2) Johnson has broken our democracy. He has ignored (trampled on) the norms and conventions that our parliamentary democracy has worked on for hundreds of years. As a consequence, we now need a much more rigorous, and enforced, set of written rules dictating how MPs and the Prime Minister must conduct themselves