A player of the same quality as Kane will not be released for under 100 million, but it may cost more. I know Kane scores too high for his age, but that's Tottenham's way of keeping Kane from going anywhere. Tottenham still really need Kane as their main striker next season, but that is not very good for Kane's achievements at the team level.
Kane is a great striker, but he hasn't won anything with Tottenham even though he is a striker who can score dozens of goals every season. So Kane needs to be able to make decisions about his future, meaning he has to force a move in order for his title ambitions to rialize.
If Kane is not traded this summer, he will go free next season. It's probably better to get 100 (90 or even 80-70) million than to lose a player for free. I doubt that a club like Tottenham can afford to "pay" for any player 100 million (plus salary) a year. But that's exactly what will happen if Kane stays for another year and then leaves - it will turn out that Tottenham will lose a player and not get 100 million.