It's a pretty obvious deduction. Unless you can prove direct revelation occurs today, what other method apart from scripture does God use to speak to his people in regards to the faith?
Do you even have a viable theory of conversion? Apparently not. Your words lead to the following dilemma. Consider the agnostic's ruminations:
"Is Christianity true? Is the Bible true? I don't know. According to Swordsman1, God certainly isn't going to speak to me about it. I guess I'll have to rely on my own human reasoning. No, I can't do that either, because Swordsman1 says "It's a pretty obvious deduction" that Scripture is the only revelation of truth. But that's the whole problem, I can't look to the Bible because I'm still trying to DECIDE whether the Bible is true. Seems I'm stuck"
Your position makes zero sense. To resolve that quandary, Calvin came up with a theory known as the Inward Witness - and pretty much every evangelical theologian in church history has agreed with him on this point. The Inward Witness is a Direct Revelation of the gospel. For example:
"No one can come to me unless the Father who sent me draws them, and I will raise them up at the last day." (John 6:44).
"My sheep listen to my voice; I know them, and they follow me." (Jn 10:27).
If God were to honor YOUR claim that exegesis is the sole means of divine-human communication, it would spell disaster for anyone incapable of reliable exegesis - they could not be saved, or at least could not be expected to SUSTAIN saving faith reliably:
(1) The majority of adolescents
(2) The mentally handicapped (viz. Alzheimers, autism, brain damage, etc)
(3) The physically handicapped (mute, deaf, blind, etc).
(4) Those too poor to receive bibles, or living in countries that ban them.
The Inward Witness cuts through all those issues - with perfect reliability. Direct Revelation is the ONLY fully reliable revelation of truth. God is a little wiser than you are. Thus while you are quick to deprecate Direct Revelation, God, in His wisdom, stakes the entire Kingdom on it.