Global consistency is expensive, both latency-wise and cost-wise. In reality most apps don't need global serializability across all objects. For instance, you probably don't need serializability across different tenants, organizations, workspaces, etc. Spanner provides serializability across all objects IIUC - so you pay for it whether you need it or not.

The other side of something like Spanner is the quorum-based latency is often optimized by adding another cache on top, which instantly defeats the original consistency guarantees. The consistency of (spanner+my_cache) is not the same as the consistency of spanner. So if we're back to app level consistency guarantees anyway, turns out the "managed" solution is only partial.

Ideally the managed db systems would have flexible consistency, allowing me to configure not just which object sets need consistency but also letting me configure caches with lag tolerance. This would let me choose trade-offs without having to implement consistent caching and other optimization tricks on top of globally consistent/serializable databases.