It's not quite correct to say there is NO IFR flight outside CAS in the US. You can certainly pick up a clearance on the ground from an uncontrolled airport in class G, and you can certainly fly at altitudes where radar cannot see you (in the mountainous regions) which might technically be below class E, but there is no notion of dropping out of a flight plan. If you lose RT in IMC (if VFR you are expected to land somewhere and sort it out) you continue on your flight planned route. Airspace will be protected for you. Even a destination procedure will be protected for you. There is a lot of class G airspace in Alaska, and I don't believe you lose your flight plan if your route of flight takes you through some of it.