8mm cartridges never faired well in this country for some reason. Look at the 8mm Mauser or the 8mm Rem Mag. Both of these cartridges are good game getters. Bullets for the 8mm can be had in weights ranging from 150 gr to 220 gr by Nolser, Speer, Hornady and Barnes. Bullets for the 338 can be had from 180 - 250 gr from the same manufacturers.
The 338 is a fine cartridge and has a reputation as an excellent game anchorer. Yes, you can get more factory loadings for this chambering but its not the only game in town. The 325WSM with proper bullet selection, will more than likely drop an animal just as well as the 338 provided everything else is equal like shot placement, bullet selection, etc.
Will the WSMs stay around? Only time will tell. These cartridges are still pups yet compared to the time tested 300 mags, 338 mag etc. We can't kill them until we have to given them time to go through their paces and see how they perform. And the only way to do that is to listen to people who have used them & relate how they performed. Paper ballistics are fine but nothing compares to actual field observation.