I guess I don't find the contrast between "scope" and "trigger" very helpful for understanding or parsing these rules. I mean, I feel that I could deploy that distinction to say that the "scope" of the Shield Master bonus action is a turn in which the Attack action is taken - and that action is taken (although not necessarily fully resolved, if I have an Extra attack) as soon as I attack on my turn.
And now there's no "nesting".
Conversely, I feel like I can insist that Cunning Action does have a trigger - to wit, when you take a turn in combat. (Ie I can't take the bonus action, effectively doubling my speed, in ordinary movement situations.) And now there is nesting.
If we can't even agree on the importance and meaning of scope and trigger as they pertain to Bonus actions, there's little point in continuing this discussion.
A turn, in terms of scope, is an abstracted time space that encompasses a set of sequential actions of one character or creature (barring a triggered Reaction). It always occurs. You can't choose to not take a turn. Even when surprised. Even when you're dying. You can't even delay your turn.
A Bonus action trigger is a specific circumstance within the abstracted space of your turn that may or may not occur. Even if the trigger occurs, it does not have to be utilized (unlike your turn).
You can argue that any Bonus action is technically nested within your turn, but your turn is not an action or an effect trigger.
So if this gets characterised as "nesting" well I just don't see what the problem is. Nesting doesn't seem to be a concept that occurs in the rules, or that one needs to explain or apply the rules. It seems to be an external concept introduced for the sake of tidiness.
Can a rogue who is Dashing as part of a cunning action drop something as s/he moves (but not at the beginning or end of the move)? I assume so - the rules don't contain a notion of "nesting" that makes me doubt it.
Movement isn't an action (i.e. there is no "Move" action defined in the rules). It is a form of currency you have that is replenished and spent on your turn at varying costs given your current condition (stunned, slowed, etc), mode (swim, fly, climb, etc), and terrain. And, IIRC, dropping an object is classified as "not an action."
And these "nesting" worries seem very similar to debates back in 2008 about the proper interpretation of the OA-triggering rules for movement in 4e (where an example in the PHB of how taking the first square of movement triggers an OA even though the whole movement action hasn't yet been resolved). I'm not sure where exactly the concern comes from - for someone who's not a programmer (ie me) it looks like maybe it's a worry about the clarity of the logic in an algorthithmic resolution process, and the possible risks of some sort of uncontrolled looping/recursion? - but I personally just don't find it a useful analytical tool in these contexts. Whereas the text and the gameplay context I find very helpful. (I'm not a programmer but I am an academic philosopher and lawyer, and when interpreting statutes text and context are the two most important things, whereas this "nesting" idea isn't part of the toolkit.)
5e combat, by the official rules, is strictly sequential except in highly limited circumstances (interrupt Reactions like Shield or Counterspell). The old interpretation of Shield Master allowed the Bonus action shove to potentially go out of scope and operate like an interrupt Reaction. It established an unintentional, undesirable precedent for Bonus actions. I'm definitely not a lawyer, but I had thought precedence in the law was often as important as interpretation.
I openly acknowledge the old rule wasn't a problem for most groups because its interpretation wasn't abused intentionally or otherwise. Continuing to use it as such won't break the game for the vast majority of people, but Crawford's clarification does fix a popular "bug" in the design and reestablishes the design scope of Bonus actions and Reactions.
EDIT: I also openly acknowledge they could have easily chose to reword the feat so it would operate off an Attack trigger on your turn (instead of Attack Action) or even drop the trigger, but the design team doesn't seem inclined to issue much in the way of errata that modifies the written rules.