Of course Jesus didn't tell his disciples to write every word he spoke...But it's clear that the written word of God is very important...'It is written' was referred to 63 times in the NT alone...The disciples, Jesus and listeners knew that the record of God was written and preserved...
Joh 20:30 And many other signs truly did Jesus in the presence of his disciples, which are not written in this book:
Joh 20:31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
Like I said, without scripture, one religion is as good, or bad as another...And obviously John agrees...
1Jo 5:13 These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
You may know, because it is written...
I agree that the written word of God is very important. But does "it is written" prove sola Scriptura? Nowhere does Jesus or any of the sacred writers say that the "Word" is limited to Scripture. In fact, Paul says just the opposite. In Romans 10:8, Paul says that the "Word" is what is "preached".