The way we trim hair for Allah after hajj or umra, similarly, polytheism also has a long tradition of using these modes as a sacrificial symbol for their false gods and deities. Prophet Muhammad (sws) forbade all such modes as a preventive measure which could have been used as a representation for any god other than Allah. This also includes Mannat for a false diety represented through such haircuts and certain Biddats related to haircuts are also known in some religious circles of people of the book at that time.
Had none of this was known to us, we would still not sacrifice principles enumerated in Quran and Sunnah in favor of individual reportings.