You're definitely more generous than I am with this whole plausibility thing. Honestly, I think it is a very weak plank to build on. First of all we're discussing Fantasy games (or similarly fantastical SF etc.). So, there is no defined way to say what is or is not possible, or even how likely it is. Gygaxian Naturalism can be seen as an attempt at an approach to this issue. The world is as it is, so presumably it is most plausible for it to continue to be that way, and some lampshades are applied to that, like describing the 'ecology' of various monsters.
Secondly, we have very little detailed information about the game world. I follow a lot of science, especially related to computational approaches to things. What I have learned is that the details matter, A LOT. So, yes, sometimes you can order things in terms of some average sense of verisimilitude that will be produced by describing things that way vs some other way, but there are always tons of roughly equally good options.
Next, the real world is filled with unusual circumstances. In fact ALMOST everything which happens is highly unlikely at some level of detail. Yet these things happen all the time, because the real world is deep and rich, it is filled with countless happenings. FOR ME at least, simply depicting 'likely stuff' isn't very convincing.
Finally, when it comes to people and social organization and what is or is not 'likely to happen' or 'plausible' in that sense, which is probably the most significant category, all bets are off. While we can characterize human beings, the truth is they are extremely complex, their interactions are highly dependent on chance events or small details that are below the level of resolution of any game world. While you might state that it is likely that the Beggar's Guild sends an assassin after you, I argue it is equally just as likely that they don't, or that they offer you a payment to go away, or whatever. And I am utterly, deeply, skeptical that a GM has any way of making an objective judgment between these options. Yet practitioners of deep world building absolutely refuse flat out, to a person, to acknowledge this!