Southgate is a classic example of a lucky manager IMO. While he has got the team to the semi-finals of the WC and the finals of the Euros, in both tournaments he has been pretty fortunate in the teams he's been drawn against to get there. If you look back on the past four seasons we have played a top 10 ranked team (plus Germany and the Netherlands) 17 times. We've won 3 of those games (1 against Belgium, 1 against Denmark, and 1 against Germany). By contrast, we've lost 9 of those games, so it's probably a decent argument that had we been drawn against a top 10 team in either of the last two tournaments we would have lost to them, as we did in both games against Belgium in the WC. In the Euros we beat a Danish team who were shorn of their best player, and a German team that is in huge transition.
The "golden generation" were pilloried for going out of tournaments to Brazil, Portugal (twice on penalties), and Germany. Maybe if they'd had some of the luck Southgate has enjoyed. Instead, he has yet to play Portugal or Argentina, and has not managed to beat Brazil, France, Italy, or Spain in 5 games.