Kompany, Otamendi, Fernandinho, D Silva, and Aguero all played significant roles this year, and they’re all the wrong side of 30.
I don’t think they’ll drop off a cliff or anything, but especially Aguero, Silva, and Fernandinho need to be effectively replaced very soon.
Silva is likely the closest to being replaced.
I don’t buy for a second that Jesus will ever be able to replace Aguero (he just doesn’t have the IQ).
Fernandinho is an interesting one. They did well with him out for parts this year, but I’m not sure how sustainable that is without a true replacement.
They're going to have to continue to retool. Aguero, Silva and Fernandinho need to be replaced, but Silva already has a replacement in house with Bernardo Silva (and to be fair, Sterling can play further back), KdB and Gundogan did ok in stretches replacing Fernandinho (but I expect they spend big on a replacement this summer), and Aguero would be a big miss, if they didn't have a ton of goals in the side, and probably a decent replacement in Jesus. I know he isn't as good, but if you figure Sane gets more matches, it's all ok in the end. Sign a good striker, and Pep will do what needs to be done. Or don't, and let Sterling play false nine.
I mean, I guess there's some question if Stones is in favor, and if they need two central defenders soon, but with Laporte, they're probably ok with one.
But a lot of their core pieces are still very young. de Bruyne is going to be 28 this summer, Kyle Walker is 28, Mahrez (28), Gundogan will be 29 early next season, but Stones (24, though he seems to be out of favor of late), Sterling (24), Laporte (24), Bernardo Silva (25 by next season), Mendy (25 next season) and Ederson (26 next season) are all built for the next 3-4 seasons.