Clearly 'Toon' is not your average RPG, so I would not disagree with you that in MOST other RPGs there is a sort of background assumption that the laws of physics as we know them would adequately describe most events, barring magic or whatever. This is, IMHO, more a matter of 'relateability' than anything else. This can be seen in certain interesting tropes that RPGs carry. For example, Traveller (and a lot of other Space Opera type RPGS) has artificial gravity. Now, we know that Hollywood LOVES artificial gravity, it just obviously makes their job feasible, but why would it exist in an RPG? There's no special effects budget to constrain scenes filled with zero-G action, yet every single location in Traveller is absolutely ASSUMED to have a 1G gravity field. The reason for this is plainly relateability, we players are used to living in a 1G gravity field, and imagining most of the action taking place in zero-G, or under heavy acceleration, etc. is simply burdensome. IMHO this is the explanation for pretty much all of this kind of thing. The game needs to work this way in order to be playable and to conform to genre tropes which originate from other mediums.
I don't think 'plausibility' is really all that much a factor. Anti-gravity, for example, is utterly implausible. As a physics-conversant person I can tell you with utter assurance that such a thing is completely unphysical and no more likely to exist in the real world than spell-casting, no matter the level of technology. So it isn't adding any plausibility to Traveller, quite the contrary! I will agree that in other genres there wouldn't be much motivation for something like gravity, or the existence of the Sun, etc. to be changed, unless you want to deliberately create a very alien sort of environment. D&D traditionally uses this technique for 'other planes of existence', and that's cool. Again though, I don't think this is due to plausibility, these other weird worlds are not really 'implausible' to any vastly higher degree than a world full of dragons and magic is.
This honestly just looks like more of the same argument. But also you are accepting the plausibility I am talking about, pointing out that RPGs often rely on deviations from plausibility for magic and exceptions, then you introduce the exception of anti-gravity, to say plausibility isn't a thing in RPGs. That doesn't make sense. Whether you file anti-gravity under a trope or an outdated theory, obviously it is operating as an exception here. But anything else in the setting is going to be assumed to conform to common sense perceptions of reality. And the anti-gravity itself is still going to be expected to abide by the players and GMs sense of gravity (unless there is something special stated about it in the rules: like it only approximates earth gravity or something).