Sola Scriptura, how can it be any other way?

I (orthodox) have been thinking a lot about this and I might be missing something and getting it all wrong. 'Sola Scriptura' is portrayed as a Christian doctrine mostly upheld by protestants, peripheral to the collectively agreed on truths of the church. Both the catholics and orthodox have problems with it. But in my understanding, how can it be any other way besides relying on the Bible completely for insight into Christianity? The Bible is the only document that gives first hand information about Christianity, the life of Jesus and his teachings. If anyone is to take the faith seriously, why would he rely on anything or anyone else apart from the Bible for questions regarding faith? For me it seems like the only correct approach is to segregate your knowledge/beliefs until they are in most accord with whatever you believe in. For example, I personally make distinctions between what church practices have biblical roots and those that dont. I have difficulty accepting the fact that so many church practices (mostly in the catholic church) are completely man-made and have no direct basis in the teachings of Jesus Christ. An objection can be made that there is an indirect link, but how can this link be enough to justify the creation of practices and beliefs, that billions of people uphold, thinking that doing these things (such as the eucharist) directly contributes to their salvation? I feel that there is an enormous burden of proof before any institution/church/person to prove that something is reasonable to believe in regards to Christianity. Please point out any fallacies in my thinking, I am just not convinced that there exists a basis to expand Christian practice and belief outside the contents of the very text of its origin.

TLDR: what is the argument against 'Sola Scriptura'?