A magnum (e.g. .300 WM,.338 WM, etc.) "can", all other things being the same, provide an advantage on large heavy boned, tough hide animals like Elk over a less powerful caliber. But is it necessary to have a successful hunt? Obviously not.
But it's just one of dozens, if not more, factors in the whole equation and does NOT guarantee success on hunting such animals.
You need the bullet weight, to hit the animal where it needs to, at the distance you need it to, with the force, weightretentionto carry through and do the damage to the internals necessary to bring the animal down quickly without making the majority of the meat into instant blood shot ground meat. Caliber you choose, can only affect some, not all of those factors.
Of course, this is assuming you've done everything else right to even spot, stalk and get the shot on the animal you need to make any of this relevant.
On the other hand, if a bigger caliber, magnum rifle is a distraction and recoil makes it difficult fora hunter to make the shot accurately, consistently, it can also be a detriment for that person. In that case, it's better to use the caliber rifle that going to give you a better chance that you have confidence in, than to buy the newest magnum that you don't have confidence when making the shot that counts. It's more about the hunter and his/her capabilities, preparation and execution than a particular caliber, within reason.