No doubt all these new super mags and short mag fad guns have cut into the current resale value of a 300 win mag. But it is in no way obsolete. I think the fascination with the ultra and super mags will fade as people get tired of the recoil, noise, and expense - not to mention the marginal gains for most hunting conditions and most hunters' shooting abilities.
On the short magnums - its hard to say what their sticking power will be. They make more sense than the ultra mags and have a lot of good points going for them. But they are so new we'll have to wait and see what their long term life will be. I'm not well enough informed on the short mags to know how they compare in magazine capacity, feed characteristics or handling 180 gr or heavier bullets.
But I 'm not dumping my 300 Win for one of the new short rounds. The old 300 win is still a very fine cartridge for all its originally intended uses. And you gun sounds like a very fine rifle.