I thought the idea was to isolate the concerns, so that you have a GitHub agent, and a Linear agent, and a Slack agent independently, and that these agents converse to solve the problem?

The monolith agent seems like a generalist which may fail to be good enough at anything. But what do I know

Say you do have those sub-agents, they will likely each have tools, and sometimes many, in which case you'll have you route to those tools somehow. The sub-agents themselves are also almost like tools from the main root agent's perspective, and there may be many of those, which you also have to route to, in which case you can use this pattern again. Put simply, sometimes increasing the hierarchy is not the right abstraction vs having many tools in one hierarchy, and thus the need for more efficient routing.