"Grognard" seems like just another label of identification that is used to justify a particular style of play and/or set of assumptions and approaches. And there's nothing wrong with that. For those who take issue with it, I would simply ask to compare it to other self-chosen identifications that you might defend, and recognize that all such "counter to the norm" or mainstream identities require some degree of "circling the wagons" or protectionism.
A grognard is someone who identifies as a grognard and, because it is no longer the dominant or mainstream approach to play, often feels they have to defend or justify it. They shouldn't have to. As a general rule, people shouldn't have to justify their identity or personal preferences, at least insofar as they do no harm to others.
Anyhow, I am reminded of vinyl fans and wonder where the line is between "I like vinyl because it sounds better, with a warmer tone" and "I like vinyl because it's old and cool." In some sense it doesn't matter, but the former is interesting in terms of asking: Which elements of grognardism are unique or beneficial to a more syncretic or post-traditional approach to gaming?
Meaning, vinyl has something that cassettes, CDs, and mp3s don't: an actual physical, auditory difference that some audiophiles (vinyl enthusiasts) enjoy. Some music listeners, such as myself, aren't tied to one particular medium, but have vinyl collections because of the whole experience: the tactile nature of the record, the crackle, the warm tone, the covers. I'm not a "vinyl purist" by any means and mostly listen to music online, but I incorporate it into my overall listening experience.
So back to the question: what is the "warm tone" of grognardism, or even simply the "tactile experience" that is unique to that style and era of play, and has been "digitized" or left behind in later eras? And can it be ported over? I think so; in fact, I think that's what "old school" is about. And why, for instance, many people like Dyson Logos's maps, or even actual polyhedral dice to random number generators. It wasn't old school back then, but it is now, and one can incorporate old school elements within a broader context that includes elements of everything that came after, and even be frame in a contemporary game structure.