The light attack weaving changes likely won't change much. Those at the very top will adjust, and remain just as far ahead as they are now. What the changes will accomplish is lower the dps of those in the middle. The people who can weave decently but not perfectly will be hurt arguably more than those who can weave perfectly, because the ones who can weave perfectly will simply tweak their rotations and setups to make up for the damage loss.

This change will likely not affect the bottom at all (if anything, it may hurt the bottom). Everyone has had the experience of joining a dungeon and having a player sit there spamming light attacks with the occasional snipe or volley. Nothing is wrong with these players; they're likely having fun and that's fine. However, thinking that a nerf to light attack weaving will even be noticeable to these players is pretty laughable.

Instead, why not make a more comprehensive tutorial on parsing? Currently, the Undaunted intro quest is abysmal. You go to a bar, drink a beer, and are then required to enter the starting zone dungeon. Why not have an Undaunted member waiting for the player in the dungeon? You could have them explain basic parsing mechanics like light attack weaving (or even heavy attacking to restore resources). You could then have the Undaunted NPC discuss the basics of a rotation; i.e., buffing up, laying down DoTs, and then using your spammable. Finally, like the pvp tutorial, you could have this whole process be skippable via a dialogue option.

Now obviously i dont know this for sure, but this would likely take far fewer resources and time than the amount of time spent balancing large combat changes in an attempt to close the gap. It would also likely have a much bigger impact on propping up the players at the bottom. At the end of the day, light attacks having a reduction in damage likely won't change much to close that gap. Teaching the players at the bottom about these mechanics (instead of expecting them to seek this information elsewhere) would be much more valuable.

TL;DR: instead of implementing (most likely unsuccessful) changes to bring down the top, implement a tutorial tied to the currently bare bones Undaunted intro in order to teach more casual players about parsing mechanics.