Unfortunatley in the UK many routes are not separated from other, even by 3 miles so leaving adjacent aircraft on own nav is simply a no no as far as the powers that be are concerned.
PRNAV routes, we will be allowed to use as and trust as they have approved that track keeping etc is all up tothe required standards and aircraft can adhere to them. However....many of these routes even with much work on the design cannot ensure lateral sep. Its just too complicated to design allowing for the combinations of where aircraft want or need to route.
So i guess its "yep" more will be left on the route to follow without instruction but some will need to be vectored or suffer step climbs. By standardising the system (ie: you will stay at lower levels longer, less early climbs and no shortcuts) more traffic can be squeezed in, but when there is weather or emergencies to intervene is incredibly complex and difficult.
Progress or not? I think the jury is still out on that at the mo.