You're not taking into account the fact the team and Lucho might just not have Cesc in the future plans, regardless of the acquisition or not of a superior talented replacement. If they simple don't count on Cesc going forward, selling him now when his value is at it's peak makes the most sense. They don't even necessarily have to replace him with an equally or more talented player. If they feel a lesser overall talent like Rakitic or Ander Herrera fits better with their vision of the team going forward, and even still if they feel those types of player would offer a better role and/or be more malleable to the coaches desires and requirements, why would it be better to keep the player just because "he is too good to let go."
Guardiola got rid of world class talent like Ronaldinho and Deco, and replaced them with what everyone would admit were inferior choices at the time in Keita and 20 year old Busquets. Why? Because he had a vision. Might as well be that Lucho thinks Rafinha and Koke or Rakitic will fit better in his vision for his project that Cesc, more talented that he may be.