The Thayan Menace said:
A destructive journey without any real beginning or end. How is that heroic?
-Samir
Just because D&D doesn't model the entirety of the hero's journey doesn't mean that the portion it
does model in unheroic. Note that the hero's journey (i.e., the monomyth) is a sum of its parts, and thus, each of those parts is intrinsically heroic. Note also that the second quarter of the hero's journey
does have a defined beginning and a defined end.
The second quarter of the hero's journey constitutes the quest or adventure, the portion of the hero's life that begins with their crossing the threshold (i.e., accepting the quest or embarking on the adventure), is characterized by tests (i.e., obstacles that the hero must overcome to complete the quest) and ends with a climax (usually a battle of some sort) that resolves the quest.
While it's true that D&D assumes already capable heroes by default and leaves determining the lasting impact of adventures upon a setting entirely up to the DM's discretion, it also very faithfully recreates the entire second leg of the hero's journey. In doing so, it's heroic. It doesn't model the whole journey, but it the part that it does model, it models very well and in a fashion that would do Homer proud.
D&D isn't
Beowulf, it's Beowulf's journey from Geatland to battle Grendel and his mother. It's not
Le Morte de Arthur, but Arthur's search for the grail and his battle with Modred. It's not the Simarillion, but the Fellowship's journey to Mount Doom and the battle with Sauron's host.
D&D isn't the whole myth -- it's the
adventure.
If one finds that D&D isn't heroic because it doesn't model the entire myth, then they must also find that Beowulf's battle with Grendel isn't heroic, that King Arthur's questing knights and his final battle with Modred wasn't heroic, and that the efforts of Frodo, Gandalf, and company weren't heroic.
That's a pill that I don't think many folks are ready to swallow.