I don't know why it took a long time for Southgate to think about replacing Harry Kane and Foden. It was clear that they were having difficulty penetrating, so what England needed was a midfielder who could tirelessly chase the ball and a runner in the front line. This was proven after playing Watkins and Palmer replacing Kane and Foden, England managed to break the deadlock.
I think Southgate is finding it hard to make the right selection in England squad, because of the number of quality players in the team. If Southgate had known how to play the England squad well, England wouldn't have been finding it difficult to score winning goals against their opponent, despite owing every of their match ball possession.
I don't know why Southgate finds it very hard to start Palmer in most England matches. He forgets that Palmer is a playmaker in matches, than Kane. Playing him in most England matches can turn around the match in England's favor.
Kane and Foden were substituted for Palmer and Watkins in 81 minutes of play, and the winning goal of England in 90+1 minutes by Watkins, assisted by Palmer
Having many quality players should make it easier for coaches to implement strategies. However, in my personal opinion, Southgate is not a reliable coach and his ideas are not that special, this can be seen in the last few matches with many quality players but very little goal productivity. However, he wasn't a bad coach either. It must be admitted that in this match he was smart in seeing the skills of his players, he knew who could be useful at certain moments, he knew who deserved to be given freedom, for example Jude, Palmer, Saka, Watkins and Toney.
And his decision to make a substitution was the right choice and he understood that in a situation like that he needed players who were simpler, and quick to make decisions like Cole Palmer who provided the assist on the second goal. Southgate also needs players whose movements are difficult for opponents to predict, and those who are willing to take risks like Watkins can be more dangerous in situations like this. Southgate understands that. England's victory puts them in the final against Spain, and in my opinion this is an ideal final because both countries have very prestigious domestic leagues.