"why does everybody here think that imgur is a failure"

Because what everyone on the internet wants is a place that will just friggin hold images and respond to HTTP requests with that image

"in light of OP's stated goals"

because op seems to want to build a sustainable business, but at a certain scale it just becomes unsustainable because you become a CDN that is paid for per-seat. So they will inevitably need to either not scale (which makes them not what the internet wants), or scale by either becoming Cloudflare or becoming a social networking site, so that they can either pay their bills via usage-based-billing or pay their bills via sweaty VC money.