does stage[123] [in|ex]clusion play a role in the "sys-" decision making? 

if the portage tree grew nodal depth of namespace rather than just the 
breadth, gentoo could be a leaner meaner vestige of its former nimble 
self by having domains of visibility and overlays of decreasing 
stability for which the sync bandwidth is committed. 

presently regardless of keywords,all clients see all packages.  if you 
don't like it, you have to run your own infrastructure.

The fringe packages will always be under-maintained but a travelling 
salesman algorithm for migrating packages on portage mirror resources 
would certainly garner a glut of proposals.