Next season Manchester City will play with the new squad because Gundongan was leave and Kovacic will replace him, I'm not really sure they will win the Champions league title. The other clubs e.g. Barcelona and Arsenal are the strong candidate now, if Bayern Munich successfully sign Harry Kane, I would include Bayern Munich to be a strong candidate too. Maybe people will include Real Madrid, but I'm not, they're in the same position when Barcelona lost Messi, Suarez and Neymar, they're in regeneration phase.
There is no need to rush to conclusions, right now there are still many possibilities that will happen at every club, just because a player leaves and someone comes to a new club doesn't mean all conditions can change. When the transfer market season is still wide open, all clubs have the same opportunity. Being a club that defends the UCL trophy is not easy, do Manchester City or Barcelona and Arsenal want to win it? all of them are entering a new phase, even Barcelona next season does not guarantee qualification for the semi-finals.
For example, Mbappe is official and finally goes to Madrid, would you include him? or still labeled with regeneration? one or two players doesn't make much of an impact, because football is all about teamwork whereas the individual is only a complementary part of it.