A system based on Scripture and Tradition alone is incomplete. You might argue that the Truth is based on long established belief and practice, but how do things get “into” Tradition in the first place?
To put it another way, how did some very ancient traditions get excluded from Sacred Tradition? Some ancient ideas and communities claim descent from the apostles, but got labelled (by whom?) as heresies.
Some ancient writings got rejected (by what authority?) from the NT, while other works, perhaps newer, got accepted.
History seems to point to a single Magisterium; not perfect in the sense of knowing everything, but reliable as to be protected from error, only on faith and morals