The scale is the problem, the assumptions you make for a 100k rps system break for a 1m rps system. For example, in a 10k rps system “just shard it and add anot

The scale is the problem, the assumptions you make for a 100k rps system break for a 1m rps system. For example, in a 10k rps system “just shard it and add another replica” might work but at 100x the throughput your replication strategy itself can break. The load isn't the only issue either you also have availability SLAs you need to meet that force you to design the system to tolerate faults at that scale or else half the internet gets taken down. Your system has to be broken up at a granularity where many engineers can work in parallel to diagnose and mitigate an issue when your service does get taken down to minimize downtime. “Scale breaks everything” as they say.