I've mentioned it before, we're very much used to playing with Rooney. It doesn't matter how crap he's been, it doesn't matter how bad any player is if you're used to playing with him you're used to playing with him. Worse still is he's part of the squad meaning he is both involved in training and has influence on the team, giving advice(likely wrong) to other players.
Like it or not he's a leader in the team and due to the way internationals work, he's been established far longer than any England manager could be. He needs to be out of the squad, having zero influence, with the England team training with the squad that will play and not looking to Rooney to set the tempo and the style of the team.
WHy people think dropping Rooney for a single game(though still heavily involved behind the scenes) would produce an instant and massive shift in style I really don't know, it's delusional to believe it would change overnight.
Remove Rooney from the squad and you still have Cahill, Hart and multiple others who feel like they know how England plays, but none of those other players were the leader or 'star' that Rooney was with the game being based around that player anywhere near as much.
Does that mean I think I saw anything promising or that Southgate is the guy to take England forwards, nope. But Southgate is probably in there to ditch some names, take the heat and make it a bit easier for the next guy to come in and stamp his authority on a team without Rooney being involved. Well that is what SHOULD be happening, I'd bet money on the next manager still picking Rooney though maybe playing him less.