Realistically, while this can work, I can say no website owner would opt for this unless it was linked to a paid backup service. And if they did, a developer/devops would have to do it. I speak from experience on that.
Fundamentally… your issues will become a question of the owner having to manage Sia and infra.
My opinion is if you focused the backup process on just IPFS (using the pinner api spec) and/or S5… and had it submit to a portal service, this would be immediately adoptable by websites. Otherwise there is just going to be a ton of hurdles that are not practical to overcome as they are managing infra.
Kudos.