I don't think there's any clearer demonstration of standards slipping than being able to say, "The Prime Minister was only fined for breaking the law once," as if this is a positive thing.
Also, to the suggestion that Johnson didn't think he was breaking the Rules and Guidelines, I suspect he didn't think so either, but as they say ignorance of the law is no defence. And this is surely especially true when you were responsible for introducing that law in the first place.
And, finally, it's worth noting that Partygate didn't finish Johnson at all. He survived it -- and, I suspect, would have been able to weather it quite easily with a more honest approach from the outset, while the Committee of Privileges wouldn't have been able to touch him if he'd started off the whole affair by refusing to comment until he'd conducted a full internal enquiry. Something to the effect of, "I can assure my Honourable Friend that I will order a full investigation into the matter of these alleged parties and I will endeavour to update the House at the earliest opportunity."
No, what led to Johnson's fall was specifically the Pincher affair, and more generally his attitude of:
a) denying everything without bothering to check whether that denial was correct (and it often turned out not to be);
b) sending out Ministers to repeat the Party line of denial;
c) changing this story sometimes only hours later, causing embarrassment for those Ministers who'd gone on air and repeated the false denials;
d) denying that steps (a)-(c) ever happened, which merely begins the cycle again.
After the Pincher affair, Johnson's ministers had evidently had enough. Being sent out repeatedly to say something that is shown to be untrue very soon after must get quite draining.