I’m on too meta now, I guess, but S3 (or any other similar one) is not too costly per se. Old images are. By stetson statistic 97 % of older images are just collecting spider web and are never shown — that storage is expensive.
And no, I don’t know how that should fix. I know what should be done, but knowing is not enough…
The bandwith costs when out there is bigger audience downloading relatively new images. Let’s forget CDNs, because when a forum is not operating really globally those new images should served from VPN itself. When images start to be older and forgotten then those should move to S3 and free some disk space.
Big guys do things differently, but they have money.