We run the Internet in a VRF to get watertight separation between management and the Internet. I do also have a CGN vrf but that one has very few routes in it (99% being subscriber management created, eg. one route per customer). Why would this create a scaling issue? If you collapse our three routing tables into one, you would have exactly the same number of routes. All we did was separate the routes into namespaces, to establish a firewall that prevents traffic to flow where it shouldn't.