I dont think anyone can say for sure, not even Big Green themselves.
It would probably take years and years of studying consumer trends to do away with the caliber altogether(its only been out for 6 years). I know its popular amongst the extra long range community, and a lot of caliber variations are made using the .338RUM casing, so that alone might keep it safe.
I do know that if you were asking this question about the .300rum, Id say you dont have to worry. It seems to be far more popular than the .338.
Personally I wouldnt worry about it. 2007 is right around the corner and if Rem still offers the caliber in their rifles(I have a feeling they will), it'll be a pretty good sign its doing well enough to warrant its stay

Another caliber that a lot of people arent aware of is the .338 Lapua Magnum, and Remmy is offering it in a Police model come this Winter(available to anyone, not just LE). If they support that caliber, they can surely support one they invented.