More than twice the storage space required for what I'm hoping will be a very large amount of images uploaded to my server.
In a collection of, say, a few million images, you will have many that generate very little traffic but others that become extremely popular in comparison. You might be worrying about "how to pay to store less popular images" and "how to pay for bandwidth for highly popular images" at the same time!
If I can publish only webp I feel like I am optimizing both storage costs and bandwidth costs at the same time. Adding more formats can help with bandwidth but can only hurt in terms of storage.