There is a strong argument for cashing in TBH. It's unlikely that Kane's stock will be higher than it is now after a 31 goal season. Teams will know about him next season and make more effort to cut off his supply lines - I think it's inevitable that he won't score as many goals next season and by definition the media will turn on him, painting a picture of just another overhyped, over rated English player that's failing to live up to his potential, even if the return is still decent but not as good.
But there's two reasons why we cannot just now.
1. Because we have a new manager going into his all important second season, having spent a year evaluating the squad and getting his methods across. Now is absolutely not the time to rip out his talisman.
2. More importantly - because our other strikers are useless. When Man Utd sold Van Nistelrooy, they had Rooney, Tevez and Ronaldo to score the goals. When Arsenal sold Henry, they had Van Persie and a young, motivated Adebayor. Who else are we supposed to rely on in the current squad?
3. We've proven completely incapable of replacing star names like for like. If there's a better alternative available at a price we can afford, chances are that Man Utd would rather sign him instead. We're absolutely not in a position to weaken this part of the team at the moment.