My comment was about the 105gr as a group, not referring only to the Amax. The Amax design adapts to slower twists for the BC available, but some 105s need a faster twist, and particularly under more demanding atmospheric conditions. True, elevation is just one factor, but all of that group will benefit from a faster twist..."stabilized" is not the same as "optimized".
But as a reality check, I doubt that more than a handful or two of these rifles in 243 over their entire production run will ever see a bullet that heavy traveling down their barrels, considering that most folks in the great unwashed see the 243 as a small game or a kid's deer rifle.
On your barrel twist cost analysis, I agree with your logical points, but manufacturers don't see things quite the same way, because the scale of their operation has a dramatic effect on their costs.
Buyers need to be careful to buy their parts in quantities large enough to achieve their price point, as buying in smaller quantities can easily double the cost and eliminate their profit.
Manufacturers are after profits, we, as consumers, usually have little concern for such mundane things.
But then, these are hunting rifles after all, and not range toys, but I bet if they fixed all their twists, there wouldn't be a dry eye in the house.
