Don't worry, I'm not stupid. I know that's a possibility, but that itself is actually an issue (you have to do it manually).
I certainly don't think a big startup could come out of this, but there's definitely an issue with our Download folders.
One of the issue with the Download folder is that most of us use it as a temporary folder, by that I meen, we download a file, open it once and then leave it to rot their for eternity.
The app could simply do things like detecting duplicates (a lot of non tech savy people download the same file multiple times) (just fingerprinting each file and then removing all versions except one), removing files which haven't be accessed for a certain amount of time (unused files), and also moving files to different folders depending on their type or source ...
I think it's just a very simple issue but something that is universally painful for every computer user tech savy or not.