My two cents.. which is about all it's worth
Against these so called lesser teams, that play the low block .. it's very important to have wide players that can take on the full back.
What this does is draw the centre back and one of the midfiielders towards the "winger" so as to cover the full back if they get beaten.
So that even if they then do come inside, the centre back and midfielder have already made a move towards that side, so either a quick switch of play, or even playing through the middle after the wide player comes inside is more effective.
At the moment with England and two wide players that always come inside, it means there's no space in the middle of the park for players like Bellingham and Foden to do anything with the ball. So for the first 70 mins against Slovenia, the defensive players could stay disiplined and there was literally no way for England to get through.
On comes Palmer, and straight away takes on his full back. This completely changes the defensive positions of the opposition, it was not about them getting tired. (although they possibly were, if there were allowed to stay in their positions they can still easily hold the defensive block!).
BTW i'm not saying it has to be Palmer and or Gordon that does this, if Saka and Foden play, then must be instructed to go take on / go outside their full back at least half the time they get the ball
If we don't play wide players against Slovakia, then I expect we'll have exactly the same problems.
Simples..