Before we do, we need to double-check what kind of referrers will be affected. But when I looked at 302 requests in T148410: Investigate source of thumbnail 302 redirects the vast majority of hits were coming for garbage blogs and websites that just copied stale html from our projects with no added value. Most of the time those pages were filled with ads. If the referrer breakdown for 200s looks the same as 302s, I see no reason to justify the ongoing expense.
The only semi-legit uses I stumbled upon (http://thewikigame.com/ http://www.buildyourmap.com/wikipediaimport.htm http://www.wikiwand.com/) can certainly work around the hotlinking limitation we would introduce. Or we could act nice and whitelist them first, reach out to ask them to stop hotlinking, then get rid of the temporary whitelist.
If legit uses that align with our mission needed this to function, I think it would manifest itself in the access log. I think that allowing hotlinking is a "failed experiment" that led to nothing other than a silent waste of donor money.