Durge and Tav
are
distinct characters in-game though.
I agree that it's likely WotC will treat Durge as the canon protagonist in any future media, but
you can find him as a separate entity in any given playthrough as Tav.
Because of that, there's literally nothing stopping WotC from treating them separately if they really wanted to.
As far as missing out on the story of the companions, I tend to disagree a bit there, at least with this game moreso than Divinity. Maybe you get some extra flavor text by playing as one of the origin characters, but their stories and respective questlines are still largely intact even with a custom character. What's more is that you lose out on that character's personality by playing them directly.
I remember trying to play as Fane in D:OS2 because people insisted you had to play as him, at least once. I gave it a shot, but it largely turned me off of Origin characters entirely.
And with games like these, "lack of story" is never even remotely an issue at all with a custom character. These games are supposed to be packed with massive, sprawling worlds that encourage exploration and interaction on a level that other genres simply don't provide. There's plenty of story to go around through the various questlines, side quest and main quest alike. And by giving the player a robust character creator, you're then giving them the freedom to infer and implement part of their own handmade story into that same game world. Just because the devs didn't do your character's story for you, doesn't mean it doesn't exist.