I think if they went this way, it would be because having Bran is a great storytelling tool to show us, the audience, the stuff we've seen (and whatever we haven't seen yet) via flashbacks. I agree it might make more sense if he's the one to tell Jon about his parentage, but if it's Sam, it doesn't mean Bran was useless. Because we were able to see it all via flashback in a way that was immersive and wasn't a BS reveal by the writers--it fit the lore of the world they and GRRM created, and created a great dramatic moment we wouldn't have gotten if Sam discovered it first. Bran is a much more effective storytelling tool for the writers to show us stuff, but that doesn't mean other characters can't learn things he has shown us on their own. Plus Bran has bigger fish to fry with the Night King and all that--it was a cool reveal but it's not like his biggest purpose as the Three Eyed Raven was to learn Jon's heritage.