If people are arguing that Henderson should be brought back to challenge De Gea, that's fine. DDG has been off his best for two seasons now so the challenge might do him good and, if not, playing Henderson might do us good. No downside, really.
If people are talking about simply handing the #1 spot to Henderson or (worse again) selling De Gea and actually depending on Henderson then that's a different matter entirely. Playing for Manchester United is an entirely different ball game to playing for a team like Sheffield United and Henderson wouldn't be the first to falter badly on that bigger stage. We can't just assume that Henderson will be a better option than even a sub-par De Gea simply based on his performances for a lesser club. He might be but he might not be, even without factoring in De Gea's own form potentially improving.