First off, no it isn't. Surviving fifteen marginal sword blows is pretty ridiculous, but it's not the same thing as causing harm to a person, by making an attack with your weapon, without actually touching them.
Putting that aside however, isn't that the rationale behind E6? It's a characteristic feature of 3e/d20 system games at least, that low level play is considerably more "realistic" than high level play. For a character at single digit levels to survive double digit numbers of hits, those hits have to be pretty demonstrably pathetic. It's not at all unrealistic to think that a trained warrior could survive twenty hits if I was the one swinging the sword (or in context, a commoner or a halfling with a strength penalty). If one believes DMG demographics, there are only a very small number of people in the world who have a high enough level to create the really wild outcomes.
This is a very different dynamic than the whole DoaM thing, wherein it's available at low levels and is actually more nonsensical given that the characters using it aren't necessarily even all that good at what they do.
Basically what you are saying is that both "D&D sword-fighting" and "DoaM" are ridiculous in terms of aligning mechanics to story... but since on a Realism scale E6 might be ranked at a '7' and DoaM a '3'... then E6 is okay to have and DoaM needs to be removed.
Whereas I'm saying that since both are on the table AT ALL... splitting hairs about where the cut-off is is ridiculous. They are BOTH unrealistic in terms of story. So at that point, I ask if the game mechanic itself is interesting and different from other game mechanics in the game. And to me... with the preponderance of "All or Nothing" rules in terms of the removal of points (you either succeed and remove points or you fail and you don't)... an "All or Some" mechanic is a nice change of pace for a specific part of the game. D&D is at its best when it use a wide variety of easy-to-understand game mechanics to interact with to keep the game interesting. And if the story or description invented to layer on top of the game mechanic doesn't interact with said mechanic with 100% "realism"? Well, its joining the club of every other mechanic/story interaction in the game.