I think the confusion exists about the role of Muftis with what some Muftis do.
About 500 years after the death of the Prophet(saw) Ijtihad was stopped due to the khaleefs opinion. I don't remember what his name was but he said Islam was complete and we don't need any more Ijtihad. Hence Muftis came about. Muftis only gave fatwas based on what was already established. They had no authority or capability to do Ijtihad.
Today some Mujtahids of whatever type have adopted the Mufti title even though they are not Muftis in the real sense of the word.
^ Muftis were around within the first 500 years so your theory doesn't fit the evidence.
Zayd ibn Thabit was called the 'mufti of Madina', so was Jabir ibn Abdullah (more specifically the 'mufti of Madina of his time'), al Yaznani was the 'mufti of Egypt in his day', al Nakha'i was the 'mufti of the Kufans'... and the list goes on. These were all well within the first 500 years (refer to their biographies in al Dhahabi's Siyar A'lam al Nubala).
After the banning of Ijtihad. The importance of a Mufti changed. There were no more Mujtahid Mutlaqs which is one of the reasons why the Most of our Ijtihad on prayer etc comes from such an early part of Islamic history.
Today the role of Mufti's is hazy, to some it is seen as the most learned scholar which can explain why some of the Mujtahideen also have the title of Mufti whilst in actuality they are not.