I'm talking about true horizontal scaling where load is sharded across replicas. Active-passive is easy. Handling web traffic is also easy. Backend business log

I'm talking about true horizontal scaling where load is sharded across replicas. Active-passive is easy. Handling web traffic is also easy. Backend business logic is not so easy. If you're message driven you can maybe have a set of replicas consume off of a message bus like Kafka or NATS, but now you need to maintain that cluster, as well as build a messaging layer. What protocol do you use, etc? Then there's the question of what triggers scaling up/down. The simple way is scale on CPU utilisation but then you get flapping issues. Or if load isn't completely evenly distributed you get one replica starving for CPU whilst the rest have too much. All of these questions go away if you can just provision a big node and be comfortable that it will be able to handle anything within reason.