So... fun fact: this is the actual dictionary definition of premium:
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It's actually simple: quality + higher priced.
The Pro line and even base this year are the "best value". No doubt. You are getting more features for less money.
But durability, material choice, and design all go to Air. You can't argue "quality" between titanium vs aluminum. And finally, Air is objectively "higher priced" per feature vs all others. So it actually fits the dictionary definition of "premium" to a tee.