With the advent of the 6.8 mm SPC, a necked down version of the 30 Remington, we will eventually see the .243 class of rounds fade in popularity. Here is why:

Military cartridges find a commercial role; the 45 70, 30 40 Krag, 30 06, 308 and .223 are in current production and will stay that way. Once commercial rifles come out with the short action, handy 6.8 rounds, we will have:

Greater bullet frontal area
Sufficient velocity and trajectory
More thump at practical hunting ranges

The 243's will fall back to varminting. And, out west where the shots can really be long, hunters will be using the bullets with the higher ballistic coeffecient anyway. (7 mm's on up.)

My prediction.