Everything is permanent–until it isn't. Twenty years ago, English football had a Big Two of Manchester United and Arsenal. Then it had a Big Four that incorporated Chelsea and Liverpool. This all made a certain sense: there were four Champions League qualifying slots: so long as the big four kept occupying them, their hegemony was self-perpetuating. But then came Manchester City and Tottenham, the Big Four became a Big Six and self-perpetuation became much tougher. The rest is that the traditional grandees of the Premier League under existential threat as a separate unit.
It has happened before. In 2015-16, when Leicester City won the league, Ronald Koeman performed miracles at Southampton, Jose Mourinho went into meltdown at Chelsea and Jurgen Klopp replaced Brendan Rodgers at Liverpool, the Big Six occupied only four of the top six positions at the end of the season.