I'd like to throw in that maybe have a set of buttons/options like you have the "track" button.
This new button would just 'reset', call this button "Uninstalled", (on the user side, not the back end) the "downloaded" banner so it doesn't show.
That way for people that try one mod, then find a mod that replaces it, they can notate that they no longer use the previous/old mod.
This would also give Nexus another data point as it can put in a date/time stamp of when a user marked the mod as no longer being used.
Possibly make it so that if a user downloads a mod, then (within 24 hours) marks it as "Uninstalled" it will not count it in the original download (A trial period if you will), if it's longer than 24 hours, then it'll keep the downloaded date/time stamp, and when "Uninstalled" is clicked it'll add an new date/time stamp to the statistics. If you want to get fancy you can even have it popup asking WHY it was uninstalled (IE: replaced with another mod, uninstalled, accidental download, etc.).
Just my two cents and the nerve racking annoyance of having a PC crash (badly), fixing the PC then having to download and reinstall games/mods and then you can't remember which mods you had because you replaced a few with ones you thought were better.