They absolutely still could have missed. Heck, maybe there wasn’t anything Southgate could have done differently that would have changed the outcome. Nothing is definite.
But it’s my opinion that he just didn’t set them to give them the best chance.
What would I have done differently?
Firstly, I would have understood the context. If you put players on with 1 minute to go, the whole world knows it’s because they will be taking penalties. You must consider the heightened pressure that immediately puts on a player. Just spare a thought for what they’d be thinking. On top of already shitting themselves because it’s such a pivotal moment for the country, you’ve now got the pressure of “everyone knows I’m only here to take a penalty. One job. What if I miss?” You’ll get the rare player who thrives in pressure and sees these opportunities as their moment to shine rather than see the fear of losing, but they are often highly experienced players. Not 19-23 year olds. And that mental strength and ability to handle pressure is rare. So what would I have done? Bring them on at, say, HT in extra time. They get their eye in, they have a chance to make a mark over and above that one specific penalty when it’s all on you.
Secondly, I’d have thought about my order some more. Ignoring that I disagree that Saka should be taking a penalty, if he is going to you must put him at a spot where the pressure is at its lightest. Which is usually spot 2 or 3 (not the first, not the one which will in and of itself cause defeat). I understand Kane at one because it’s key. But your next most “ready” player has to be at 5. Think of it this way, Maguire didn’t put in a brilliant penalty because he is technically better at them then Rashford or Sancho. He executed it because he had better control under the pressure (in my view).
For me, lastly, I wouldn’t have picked players based on who does the best at them in training. For me, I’d have based it on who has confidence. Who has faced adversity before. Who he knows can handle pressure. But this is hugely judgemental. Maybe those 3 were those players. I’d be surprised, but it’s possible. So I’m less focussed on this.
Like I said, anyone can miss. And maybe they would have anyway. But in already intense situation, the timing and order, in my opinion, further heightened the pressure on these young lads and without that extra burden, who knows? Maybe Rashfords shot is a few CM to the right and goes into the goal. We will never know.