I do not think that the question of the next season is relevant at the moment - everything should be decided now. If Kane leaves for free, then Tottenham will be in a big loss, so if Kane's departure is a definite thing, and the club knows it, then they are obliged to sell him now. The situation is similar with Mbappe, so I hope to see these two transfers now and not in a year.
Tottenham have not mandated him to sign a new contract for now. Maybe they aim to keep him for one more season because he will be useful for them to compete for the league trophy or secure a champion league slot. But if the club wants to consider the business part of the deal, the best option will be to make him extend his contract or sell him this season. Besides these two options, Tottenham will lose close to €100m if they let him leave for free next season.
Yes, it's simple math: having Kane doesn't guarantee anything (8th place lol with Kane scoring 30 goals) and his departure means that the rest of the year will cost Tottenham 100 million (which they can now get for him) plus his salary. If Tottenham had the financial capacity of the City, then maybe it would be normal to use his services for another year at this price, but for Tottenham this is a very minus option.