"Not supported by the written word of God" amounts to someone's opinion. I think they are supported by the written word of God, but people can explain away every Scripture I cite to support any Catholic belief, because that's how they get to believe what they want.
And that's just a specific case of the more general problem that people can explain away every Scripture cited to support any Christian belief, which is why you have liberal Protestants who can convince themselves that the Bible endorses e.g., homosexual behavior.
How did the early Christian leaders defend the tenets of the rule of faith against the false teachers of their day? By asserting that the "church" believed it, therefore it was the truth or was it because they could show that Divine revelation clearly taught it? I say it's the latter but that does not rule out the real place and purpose of Tradition. The Christian assembly is tasked with upholding and supporting the truth as God has revealed it. That does not give the church license to invent (develop?) doctrines that were not taught by God's word and make them binding upon all Christians under threat of excommunication - that's how cults operate.