Reading this port-mortem / plan shocks me, this doesn't look like a service that has been serving high-throughput services for more than a decade. In fact it is almost like they've barely started. It seems the solution has been capacity, capacity rather than architectural or data changes.
> Our next milestone is an architecture that scales read capacity linearly with the number of readers, enabling unlimited read operations
How do you not have read-replicas / read caches at this scale yet? Which is what I am reading from this statement. You can of course get really far with sharding and whatnot. But at some point it might become worth it to engineer your data into a model that scales better.