The light really IS getting brighter, but what kind? Which one?
I can think of three types of figurative light in this context:
1) That which JWs refer to and they base on Pr 4:18. Light on a path... light that gets brighter. They say that their understanding of the Bible gets better with passing time. "We have new light on this point." However, Proverbs chapter 4 is not about doctrine, and there is no evidence that any doctrinal light has gotten brighter for JWs. They've changed doctrine many times, but no one can show that the new teachings are more accurate than the older ones. That would have to be proved in order for them to claim evidence that there is doctrinal light has gotten brighter.
2) The light that should emanate from those who have truth and properly represent God. Mt 5:15,16: "Neither do people light a lamp and put it under a bowl. Instead they put it on its stand, and it gives light to everyone in the house. In the same way, let your light shine before others that they may see your good deeds and glorify your Father in heaven." For a while, in years past, it seemed that a light was emanating from JWdom and individual JWs as they seemed bold in their ministry and willing to take on challenges. Now they run from challenges with their tails tucked between their legs. They are doctrinally weak and they are no longer bold and courageous. So, this light has actually become far weaker.
3) The light that shines on JWdom. I can't think of any Scriptural reference to this type of figurative light, but it does exist. We can say that as we gain evidence about and learn more about some thing or entity, the light shining on it gets brighter. For example, if we are investigating a murder, at first, we might just have a dead body. There is very little light on the situation, but, as information is obtained, the light gets brighter and the situation can be seen far more clearly.
Think about if from another angle. Criminals, deceivers, cheaters, etc. love the cover of darkness; they don't like light. They don't want light shining on them. JWdom is like that. It doesn't want light; it doesn't want scrutiny; it doesn't want to be analyzed. It wants JWs and others to just accept what the leaders say and to research and study only in JW approved materials.
Before the internet, JWs could, in effect, operate in darkness (with some exceptions). Then came the great bright light of the internet. JWdom hated it and resisted it at first. It then realized that it had to selectively embrace it. The internet has seriously wounded JWdom. It has shone a great light on it that is getting brighter as more and more the failures, flaws, deceptions, weakness, etc. are seen in greater detail with the increasing light. Yes, in the last two decades or so, the light that shines on JWdom (primarily due to the internet) has become brighter and brighter.
If JWs were what they claim to be, they would love the exposure. They would love for something like the internet to come along and shine a great light on them so people can see what they really are, but they hate the exposure. Thus, there's no way they can be what they claim to be. They aren't lovers of light; they're lovers of darkness.