Both the auto-upload and fetch apis are perfect for use cases where you are proscribed from serving images from an origin domain.
When building an MLS listing site, my team was grateful to be spared what we thought would be a significant development effort.
When site expanded to a nation-wide inventory, our Cloudinary costs went from several hundred dollars per month to over $2,500!. They were projected to hit $5000 per month before long.
The current solution works perfectly with Cloudflare and gcloud and reproduces much of the auto-upload api, including image resizing. You can read the details on the github repo, but I can imagine the basic solution supporting a number of different "recipes", like different storage mechanisms and cdns.
If interest in the project was high, I could be persuaded to work on it.