The use case I always think of is the developer experience for regular hobbyist and workaday devs writing their apps with local-first sync.

Apple comes close with CloudKit, in that it takes the backend service and makes it generic, basically making it an OS platform API, backed by Apple's own cloud. Basically cloud and app decoupled. But, the fundamental issue remains, in that it's proprietary and only available on Apple devices.

An open source Firebase/CloudKit-like storage API that requires no cloud service, works by p2p sync, with awesome DX that is friendly to regular developers, would be the holy grail for this one.

Dealing with eventually consistent data models is not so unusual these days, even for devs working on traditional cloud SAAS systems, since clouds are distributed systems themselves.

I would be very happy to see such a thing built on top of Iroh (a p2p network layer, with all the NAT hole punching, tunnelling and addressing solved for you) for example, with great mobile-first support. https://github.com/n0-computer/iroh

Off topic, but why would I choose iroh instead of libp2p which seems to have much better language coverage compared to iroh?

Your holy grail is probably Fireproof: https://fireproof.storage/

Open source, uses object storage without a web server dependency, syncs, and has great DX.