From the PlanetScale perspective keep in mind the ability to shard. What happens when the largest single node Aurora instance can no longer keep up with application/traffic demands?

I ask because we see it more often than not, and for that situation sharding the workflow is the best answer. Why have one MySQL instance responding to request when you could have 2,4,8...128, etc MySQL instances responding as a single database instance? They also have the ability to vertically scale each of the shards in that database as it's needed.