I think the key thing from a narrative and design perspective is that impact lines aren't obscuring the background – because for one narrative beat nothing in the background is important in that area. The reason that they are effective is that human consciousness focuses on moments of impact, of specific imagery which communicates a single important second, and all else fades away. This is why people are notoriously bad at reporting what has actually gone on around a significant event, because their consciousness is focused at that single beat.

Doing that within a paneled framework makes use of that subjective experience that everyone has had.

So, for example, if we take the Captain America page that you reproduced and highlighted, look at the composition of the panels where significant impacts occur. It's important to note that even if there were no impact lines, the background wouldn't convey any more information than we already have from previous panels. In the first, all that would be there would be the chest of the Leaper. In the second, all that would be there is Captain America crouched and uncoiling. These are things that we already know exist in that narrative; the only thing that's important is first the uppercut and then the Roundhouse. That and the agonized expression that says how effective those impacts are.

It's absolutely true that impact lines are a purely abstract representation, and as such, certain methods of storytelling in graphic art are more suited for using them than others, but seeing them as replacing or leaving out background isn't how they are usually presented. They are, instead, probably better thought of as ways to focus the attention of the viewer on a specific moment for narrative purposes, just like using a shallow depth of field shot in cinema in order to focus the viewer's eye on action that's occurring at a specific point.