Don't see why people are still arguing about De Gea versus Henderson. That's over and De Gea won out, so the comparison isn't really relevant any more. He won't be displacing De Gea.
The question is what happens when De Gea leaves, hopefully this summer. Would you want to give Henderson another chance as #1? Or would you want to spend money to sign someone else? So the relevant comparison is Henderson versus all goalkeepers who could likely be available. Within the PL for example, how does he compare to the likes of Sanchez, Raya, Meslier, etc?
And that's assuming bridges haven't been burned between us and Henderson anyway, which may well be the case. Because if they have then it's just a question of how much we can sell him for.