You're right we could e.g sign Haarland only for him to miss two seasons with injury and never be the same player similar to Mendy (though Mendy was pretty injury prone even in France..). But like Kompany, Aguero and now De Bruyne it feels like a guarantee that Kane'll be out for parts of the season, only question is when and how long. And similar to those players he takes a few games of looking very rusty to get back to speed, they're too big to leave out so get shoehorned into the side regardless of how things were going in their absence which causes a lot of upheaval.
Have seen suggestions that it won't be an issuse as we wouldn't run him into the ground like Spurs do but not sure about that, if he's the only forward we sign we'll still be pretty reliant on him and Kane doesn't seem like the type to accept the rotation like a younger player might.
Pep I think had Aguero on a vegan diet which reduced the rate of injuries but robbed him of most of his explosiveness, maybe that's part of our thinking with Kane who doesn't have much of it to lose.