Yes, in fact, Tottenham have already lost Kane since he himself wants to leave, so it's better to get money for him than nothing at all. And if you think cynically, then Tottenham is already at the bottom (8th place and no games in European competition) and things will not get worse for them if they try to change something. By selling Kane for 90 million, they can buy someone for 40-50 and hope for Hong Si Min and also for Richarlison to start playing.
Ahead of the start of next season's Premier League it seems that most players are trying to make several options for their careers. I see that Kane is really fed up and he can't lie to himself anymore by staying with Tottenham, he's starting to realize that as long as he's with Tottenham he can't get anything even the trophies he really wants. No matter how good the quality of his players, he will not be able to get anything if he is in the wrong team. Then for Tottenham itself I think Postecoglou realizes that Kane no longer wants to defend himself there anymore, and I hope that the Spurs management will make the decision to sell him as soon as possible if they don't want to suffer losses from Kane's departure. and then look for other alternatives using the funds from the sale of Kane like you said.
Even so, Manchester City will not immediately become weak next season. Because Manchester City are very prepared for the transition phase they are currently experiencing, so there is still a possibility that Manchester City will continue to be strong next season, even though there might be a difference from last season when they won the championship. And in terms of substitute players, indeed it must still be there in order to anticipate unexpected things like you said that.
I think before Guardiola was willing to let go of some important players like Gundogan I think he has thought about all the consequences and maybe he already knows what he has to do for next season's competition. We all know that Manchester City won the Premier League last season and Guardiola certainly knows that all clubs, especially Arsenal, will not make it easy for Manchester City next season. Therefore, I think Guardiola has thought of something to anticipate the various threats they will face.