The duration of the attack action matters because we need to know how long it lasts in order to determine when we have met the condition to bonus action shove. Hasn't that always been the obvious reason to care about duration in this conversation?
What your saying isn't simple. The same arguments about duration can be made in your system, its just duration is now duration is concerned with a single sequential element vs multiple sequential elements. Instead of talking of duration as number of seconds we can talk of it as number of discrete elements. So there's no escaping the concept of duration. It's just as present in your formulation as in the other, whether or not you want to talk about it.
The Attack action starts as one discrete element. The rules say you can break this action into separate pieces and insert other (edit: specific types of) discrete elements in between. The Attack action is still just those N pieces, and is complete once those N pieces have been processed or resolved.
Example: Move, Attack, Move. If you have 3 attacks from Extra Attack, then your turn might be moving your miniature, making 3 attack rolls against your target, and then moving again after you kill it. The Attack action is clearly over once you've made those 3 attacks.
Example: Move, Attack, Move, Attack, Move. If you have 3 attacks from Extra Attack, then your turn might be moving your miniature, making 2 attack rolls against a target, moving to a new target since you killed that one, making 1 attack roll against a new target, and then moving some more after you kill that one as well. The Attack action is clearly over once you've made those 3 attacks. We don't care that you moved in between, your Attack action is still those 3 attacks in 2 discrete elements on your turn.
The "If you X, you can Y" triggering translates to this concept in a very straight forward manner. Before you add a Y element to the list, X must be completed. If X is the Attack action and you split your Attack action into 2 discrete elements so you could move between attacks, then Y must come after those 2 Attack action elements in the list. Example: Move, Attack, Move, Attack, Move, Shield Master Shove. In a nutshell, if something triggers from the Attack action, then you just don't get to do the Attack action (or pieces thereof) after you've done the triggered bonus action.
Or, if we want to really simplify this: No part of the triggering condition can come after the triggered event. If the trigger is the Attack action, then it makes no sense that you can perform parts of the Attack action after the triggered bonus action.