I have some trouble understanding what people mean by scalable today, especially why people seem to have to run entirely event driven, and not mostly on the soc

I have some trouble understanding what people mean by scalable today, especially why people seem to have to run entirely event driven, and not mostly on the socket read/write edges? Soon to be almost 20 years ago we pushed >10k messages per second on a JVM, using essentially pentium pro class hardware, and the messages spread over thousands of TCP consumers, yielding average latencies well below 0.5 seconds. Not really low latency, but low enough that we didn't need much lower This was on a purely blocking implementation, because that was before almost anyone did anything like that on the JVM. With the advancement in async IO, it's got to be possible to drive many millions of sockets, or have really low latency targets before you have to start being really careful? So what are you guys doing that seems to need so much async code? With that I mean actual async code, not code having locks but pretending to be async by using callbacks everywhere, because that's somewhat common. I'm not trying go be rude, I honestly don't get what people are doing that needs more than the JVM should rather easily provide, unless possibly you have super low latency targets? There seems to be too many that have to resort to quite cumbersome implementation strategies, so I'm starting to think there's some corner of the industry which I have completely missed, and which requires these strategies regularly?