Look people, i know this might be hard for you to understand because you happened to have an absolute genius in charge for 25 years, but football is an environment where change is needed periodically to keep things from going stale
Capello just yesterday mentioned how in his own experience, it's impossible to coach the same team for more than 5 years - by year 5 players stop listening, because you're saying the same things you've told them for years. Motivating and getting through to them becomes impossible
People keep mentioning Klopp at Dortmund. Well Klopp left Dortmund precisely because he realized just that: that had he stayed on, they would have needed to change half the players. So he did the best thing for the club, he left, a new manager came in and with those same players Dortmund went from a moribund side to a side capable of actually challenging Guardiola's best bayern side in the league
Spurs needed to make bigger changes to their team in the summer, or change manager, because Pochettino's cycle with this particular group is over.