There's a middle-ground between bookmarks ("permanent" storage where it feels like you're archiving the page), live tabs (obvious issues), and "to read" lists (which are better for a stream of content that you know you want to read but can't get to yet), that isn't served by current approaches. Firefox Preview (Android) had a really good idea with "collections", where you essentially save tabs into groups that were visible on the "new tab" page, but it doesn't seem to have percolated to any of Mozilla's other browsers.

My painful tab overuse is caused by skimming HN/reddit a few times a day, popping anything I'm interested in looking at into a background tab, then getting to those tabs as I go through the day. It doesn't feel the Pile Of Prospects should be in a "to read" list, as it seems that such a list should only contain long articles & videos & etc that I'm certain I want to read but don't have time for. That is, the "to read" list is meant to be something that could be integrated with a tool like Pocket, where the content could be backed up and read offline while in a continuous, focused state. Rather, the Pile Of Prospects should go in a sort of "haven't looked at yet" list that needs to be pruned and which is always visible so as not to go untended.

I'll probably write an extension to deal with my use case, but I think this slow creep is how most people end up with a thousand tabs open.

I do agree that thousands of open tabs is a terrible way of operating. It's not great for memory consumption (even with tab discarding), it makes hunting through the "real" active set of tabs difficult, and people do end up losing their Pile Of Prospects. I'd love to see Firefox et al gain better support for this without needing to resort to extensions, especially as most extensions designed for this are a touch too slow, have rough edges, or try to do too much.