There was never a fixed top 4 though, was there? First it was Man Utd, Chelsea, Arsenal and Liverpool, then Spurs for a year, then City for a longer period. It was always dynamic. Spurs have been thereabouts since about 2006.The mere fact of his age made it likely that he would soon retire for one reason or another. It's not rocket science.
In case, it's not all about United and Fergie. The signs of increasing top 4 volatility have been there to be seen - except those for those who insisted on burying their heads in the sand and claiming that the composition of the top 4 of was more or less cemented in place for good. And the signs of a rising ambition from Spurs were also straws in the wind.