The one big box assumes that you know how to configure everything for high performance. I suspect that skill has been lost, for the most part.
You really need to tweak the TCP/IP stack, buffer sizes, and various other things to get everything to work really well under heavy load. I'm not sure if the various sites that used to talk about this have been updated in the last decade or so, because I don't do that anymore.
I mean, you'll run out of file descriptors pretty quickly if you try to handle a few hundred simultaneous connections. Doesn't matter how big your box is at that point.