I think you can allocate for injuries. Last season was unique in that we had a crop of players who were all at or reaching their peak, helped through with a sprinkling of youngsters. To have so many go through unscathed was quite an achievement, yet that is simply unsustainable (physically and psychologically) every year. With the Euros and Copa America, we can assume that there will be injuries again next season.
The Foden/de Bruyne question is an interesting one, but I’m unconvinced that it is an either/or. Foden comes inside and gets goals, which everyone enjoys, but de Bruyne has 14 assists in 1,130 minutes and Foden has 10 in 3,680, the vast majority of which came when he played wide. Getting the maximum from both is the challenge Guardiola faces.