I already heard one argument on this, that the Scripture referred to (Proverbs 4:18) in context was actually advice of a father to his son on how during his life he would continue to grow spiritually throughout his life.
But a closer look shows that there is no mention at all of new light, in any context. It says "but the path of the righteous ones is like the bright light that is getting lighter and lighter until the day is firmly estblished" [analogy: from darkness of the night gradually to the full light of the day?]
The Society's "new" light repeatedly reveals that what they previously taught was false. Until 1980, organ transplants were forbidden, this was either true or false teaching. Then the Governing Body reversed its decision, was this truth building upon truth, or a reversal in doctrine?
Even the old watchtower taught there could be more clarity, or fuller understanding of a teaching, but not a reversal of that teaching.
Sorry if this is obvious, but I'm new to all this, and although I've read that verse several times before, the truth of it just hit me!