The England squad has so many star players that there are enough for 2-3 national teams by a large margin and the fact that Cole Palmer is losing the competition and not making the squad proves it.
As long as Southgate's tactics give results everyone tolerates it, but what will happen if Southgate's tactics when scoring one goal the team starts to slow down and park the bus in the penalty area will not work? I think given the selection of players Southgate will not forgive the loss, because as you correctly noted England has a real star team, all the members of the best clubs that play in the Champions League.
I wonder how they feel themselves who have to play such football, which they are not used to, because Arsenal and Manchester City and other top clubs play attacking football.
England didn't play to keep the score, that's not their style, at least not against such an opponent, Serbia only made a few shots on target, it was difficult for them to play against England. It is difficult to blame Southgate for a bad strategy as long as they are winning, after all, the main thing is to get 3 points for a victory, and not to achieve a crushing victory. Although Bellingham's early goal hinted that England could score much more in this match, as I said, I don't think this is an important indicator, the most important thing is that the team wins.
I think this could be made into a motto for the current England team "It is difficult to blame Southgate for a bad strategy as long as they are winning, after all, the main thing is to get 3 points for a victory, and not to achieve a crushing victory.". I think Southgate would totally agree with you that the main thing is to get results and the game is secondary. This point of view can also be, but unfortunately very often we remember just the highlights and matches and the England national team when Lampard, Gerrard or Beckham played for it, although they did not play so effectively, but liked everyone much more and remembered better than Southgate's current team.
In any case, time and strong opponents will show whether Southgate is right to use this strategy.