It's one thing to be able to scale database compute/storage; it's another thing to be able to partition it. It's extremely common for bad queries/access patterns to cause noisy-neighbor effects on other simultaneous accesses to the database, to the extreme of knocking the whole database over with timeouts/OOMs/etc.

Scaling out DB compute can only help with that to a (expensive) point; eventually, you end up wanting to either prevent the bad queries from being added to the system (DBA culture) or ensure that the bad query runs on database infrastructure that doesn't affect other queries. That's why partitioning DB compute (and storage: noisy-neighbor effects from a bad query at the storage layer don't require storage to be running e.g. a BookKeeper or whatever on a server; they can manifest as hot S3 keys or cloud object/block store rate limiting) is a necessary capability if your plan for dealing with a culture of "anyone can add any access pattern they want" is to scale the DB.