Replacing the chocobo companion with retainers would only make the chocobo obsolete. From the system you described, the retainers are simply better in every way! This trivializes the time and effort players spent on leveling their chocobos. Hence, I am very much against the idea laid out in the OP. However, if the intent is simply to do a skin swap (replacing the chocobo with a retainer through a cosmetic swap), then I am agreeable to such cosmetic swap.