I am sorry, but Obertan and Diouf left in the 11/12 season, so they cant be used to illustrate poor succession planning. High profile signings are not always the priority. Balance is the overarching key factor.
In the three seasons leading up to his final season, SAF bought Bebe, Smalling, Chicharito, De Gea, Jones, Young, RVP, Kagawa, Zaha, Powell and Buttner.
From that list, only RVP was not young and future focused. Jones, Smalling, De Gea and Young are still here. Chicharito and Kagawa were still around when he left, but subsequent managers sold them. Zaha and Powell had potential to still be around, but couldnt reach that level.
To even think that SAF was not planning for succession or the future is crazy. He was constantly ensuring that the squad had a balance of quality, potential, youth and experience. It is never going to be a smooth transition and the fact that he won the league in his final year with a balanced squad, shows that it was not due to his planning that the subsequent managers were poor.