In the process of gathering the required data from 가이섬.com for that thread, I also grabbed Melon's weekly unique listener counts for each song and inserted them into my Excel spreadsheet.
From there, it was simple to calculate the average (both mean and median) number of weekly unique listeners for the top 10 songs in a given week, and then graph the results. Again, my numbers cover the period from 2018-11-26 to 2021-05-10, while the vertical red dashed line marks the date of Melon's chart reform (2020-07-06).
As we can see, the number of unique listeners rises and falls quite a bit. However, the general trend is definitely a decrease in ULs, which is consistent with reports of Melon losing market share in recent years. In addition, it does not appear that chart reform has affected unique listener counts significantly.