The S3 model means content creator pays for bandwidth, content consumer pays nothing. For the use cases that imgur and similar image hosts provide, that wouldn't work. And as you mention, that's high friction for the uploader.

I don't have a solution to the problem other than big pocket provider offers it as a public service. Somewhat like Let's Encrypt, or Cloudflare.

Edit: I suppose somebody could make a slick interface that uses parasite type storage and redirects...stealing storage from various free services like Google Drive. Offer some kind of karma points for creating and donating new Google Drive, OneDrive, etc, credentials. Would likely get shut down though.