And God used Harrod, Judas and an ass to fulfill His will. So what’s your point?
They would argue that Rome was not simply used to fulfill God's purposes, but was the instrument and steward of Scripture (the NT) and and as the inheritor of promises of Divine presence and preservation, she only has historical descent as the steward of Divine revelation.
But not only, but as writings are of God, and what they mean, are both subject to interpretation (thus 10 billion Prot. churches), then an infallible, incontestable magisterium was/is required, and promised, to whom all must submit, and in the light of history etc., then Rome is it.
Without even arguing against the church of Rome today being essentially the same as the of Scripture, can you (as RCs have not) tell me what is wrong with the premise behind this stewardship=infallibility/incontestable authority argument?