The lack of a truly first-rate 6 is a huge problem. Here is who nailed that down in the final for four of the last five teams to win a World Cup or the Euros: Busquets, Schweinsteiger, Kante, Jorginho. All world class players with a Champions League winner's medal - the sort of players you want destroying people while linking defense with attack. Having someone like that back there isn't quite a necessary condition for lifting a major international trophy, but it's close.
Behind them you have Stones and Maguire, who are the rich man's version of our Keane and Holgate pairing. There's always a horror show waiting to come out, like when they both totally switched off mentally and gifted Mandzukic the space that put Croatia through. They're better players than ours, to be sure, so it doesn't happen as often. When it does, as it did on Hungary's last goal, it isn't pretty.
One could argue that Southgate's timid setups in the big losses were simply an attempt to paper over those cracks. Of course, one could equally argue that the best defense in that situation was to put the opponents on the back foot, rather than try and fend off their attacks for ninety minutes (accounting for added time) after early goals.
It's the $64,000 question, since he just won't try it. If the manager was Roberto Martinez, we would know. You might have gone out 5-3 in a knockout fixture before the semifinals, but we would know.