Kane has served Tottenham enough, at the age of 29 he has to look for challenges at other clubs to win trophies that have never been won while he was at Tottenham. Manchester United's interest in him is the biggest opportunity to play for a bigger club than Tottenham, at Manchester United he has an easier chance of getting a trophy than at Tottenham.
Tottenham are still fighting for a fourth-place finish in the EPL standings, but if Tottenham fail to finish in the Champions League zone, Kane will have to take a stand to get a chance to play in the Champions League.
Indeed Kane has sacrificed a lot in his career for Tottenham, good things he has also achieved a milestone for himself at Tottenham having surpassing the record of most premier league goals and Tottenham's all time scorer which sums up how much he has given to the club. At this point leaving Tottenham is the best option in my opinion and it will be a big move switching to Manchester United where he can achieve a lot.
He seemed to find it difficult to do that even though with his current condition, of course, things like this were still much easier than him having to continue to torture himself and his desires in wasted trophies.
At the moment there are a lot of better clubs who want his services and it's definitely the last hope for a move because if he stays and his contract is extended I think in the future it will be difficult for him to find a club when his form drops.