Except that Southgate actually has the players to play everyone in their correct positions, and instead, plays them out of position to fit in another player (TAA) out of position.
You could easily play:
Pickford
Walker------Stones-----Guehi------Trippier
Rice------Bellingham
Saka------Foden------Gordon
Kane
And Trippier would be the only compromise. Bellingham is arguably the closest thing England have got to Gundogan. He's decent enough defensively, fairly physical, and incredible at arriving in the box at the right moment. And Gundo spent his entire City career playing behind Silva, De Bruyne or Foden. The reality is that that formation above would already be pretty defensively solid, because Walker and Trippier are not going to be joining the attack that much anyway.
Ignoring Foden's performance, let's actually look at how many chances England created. They had 5 shots in 90 minutes, so it's not like Bellingham and "Trent" have shown themselves to be creative geniuses. Most of the threat came from Saka, which is normal, because he's the best player at taking on opponents one-on-one, and in the absence of any coherent attacking plan, that's the best way to make something happen.
Absolutely Foden hasn't been good enough, but if you're handed the best player in the Premier League last season, the best player in La Liga, and the highest scoring striker in Europe (and it's not like Saka is a weak link), and you manage 5 shots on goal against Serbia, then maybe you need to look at the manager and the tactics. For comparison, the Netherlands managed 21 shots, Switzerland 15, Spain 11, Croatia 16, Italy 17, Germany 20. Even most of the losing teams had more shots than us.
We hear a lot about how the defence is weak so we need to set up to protect them. Walker was in almost everyone's team of the season, and is a good shout for best RB in the world. Stones had an injury-hit season, but he's still the player that a year ago was given credit for transforming us into treble winners with his intelligent play, and being the best player in the CL final. Trippier too might be getting on a bit and playing out of position, but he's a quality, experienced defender. So what they're basically saying is that we need to constantly build our entire team around protecting the back four because one of the four isn't a CL regular or playing for a title-challenging team.