Thank you for your suggestions! I will given them some serious consideration. Some quick feedback right off the bat:
1. Right-click the page and choose "Put the page on the Pile and close it".

2. This might not match the work style of other users (some songs/videos/docs want to be heard/seen/read several times before deletion).

3+4. I am trying to keep Pile as lightweight as possible - performance and UI complexity-wise. Powerful search and filter functionalities are already provided via Bookmarks/Show All Bookmarks. I do not want to re-implement this and cram it into the sidebar. But I do understand that piles can become overwhelmingly large over time. I will meditate on this.