I respect your opinion, but the ballistic data and evidence is on my side on this one.
He said he already has a long range rifle that he doesn't want to carry around all the time. This is a rifle for hiking around and exploring.
If faced with a 250yd shot and a 7.62x39mm (or, say, a 30-30), he is in the same position as if he had a 6.5x55 and a 500yd shot. Every caliber has limits.
If he is using open sights (which we both agree are advantageous in the bush), chances are that 250yd shot won't be a realistic option anyway. I know it CAN be done, but I believe it's beyond most hunters.
I agree there are more versatile rounds out there, but I contend that within the realistic 75-100yd shots he might be making while exploring and hiking, a 30-30 (or equivalent caliber) would be a good choice.
Plus, with the cheap ammo available, he might be more inclined to practice with it often and get to know the rifle well.
We might just fundamentally disagree, and that's ok too.
I read the OP's question in terms of a general purpose big game rifle for the Yukon. The general purpose nature of a big game rifle requires near range and moderate range performance, on a broad spectrum of game whose weight might be 100 pounds or might be more than a ton. Clearly 300 yards is not long range, since the rifle can be sighted in so there is no need to hold for range, near or far, on a big game target. If its within the maximum point blank range, its not long range. For this purpose, any of the full sized service cartridges loaded with moderately heavy for caliber bullets that have a minimum velocity of 2500 fps will suffice quite nicely. Certainly opportunities for a moderate range shots occur between 200 and 300 yards when hunting in the tundra, the mountains, across burns, and along lake shores, and the thing about bush hunting in many areas across the country, not just the Yukon, is that thick bush can rapidly open due to burns, lake shores, muskeg and tundra. While I contend that iron sights are a practical feature to have on a rifle, in low light they are no match for the light gathering qualities of a variable low power scope, particularly a scope that has an illuminated reticle. On a clear bright day, iron sights would be of little use on a 150 yard target standing in the shadow of a rock or a large tree, you simply would be unable to see it; suffice to say that versatility of the rifle is enhanced with an appropriate optic.
None of this is to say that the 7.62X39 has no place in the outdoors. I believe a 7.62X39 CZ-527 would be a dandy little rifle in areas where short range and game weights under 500 pounds are the norm. A friend of mine shot a respectable moose here with his Ruger 77 in 7.62x39 and it would have died no faster had it been hit with a .300 magnum. That one experience though doesn't make the little rifle a moose gun for just anyone, primarily because the circumstances of that particular situation could not be counted on in most big game encounters. Additionally, here as in the Yukon, the kill might be contested by a large bear, white or brown, and even if you are willing to give way to the bear, he might choose not to provide you with the same courtesy. In such circumstances the little CZ carbine wouldn't provide me with the same warm fuzzy feeling as one of my larger rifles, whereas even my 6.5 Krag would suffice.
As for inexpensive shooting being beneficial to the hunter, I absolutely agree with you there. This is why I champion .22 centerfires, particularly the .223, cast bullet handloads, and Mexican match loads for larger rounds. But while it may improve marksmanship, frequent practice with a 7.62X39 doesn't make it a better big game cartridge, it is what it is. A .30/06 (.308/180 @ 2800 fps) exceeds the velocity of the 7.62X39 (.308/150 @ 2100 fps) by 700 fps with 30 grs more bullet weight. There simply is no correlation between the practical range and the terminal performance of the two rounds. Even the 6.5X55 load which drives a 140 gr bullet at 2700 fps, outclasses the 150 gr load in the 7.62X39. I keep referring to the 150 gr 7.62X39 load simply because I believe it to be the best game load with that cartridge.