I don't think the problem is that only one class matters in a meta.
It comes down to how they want to make the core stats asymmetrical, but can't sufficiently make them equivalent in results with different methods of power. Mind you that the stats have always been asymmetrical, but riot has been attempting to change the lines by which they differ. Resulting in a disparity, and noticeable favoritism, in the designs of AD itemization and usage.
This is a problem of their own creation. They kept pushing changes into the game, as if there existed no negative long term consequences for the changes in design philosophy. The largest of which is probably the shift of AD into the role of AP, by their very names. Attack Damage is being treated as "physical damage" applicable to both abilities and auto attacks, but AP is still heavily restricted to magic damage, rather than expanding to include all forms of power that abilities offer including utilities like range and CC (beyond an experiment with a few supports a few years back which riot didn't expand upon).
If they wanted to expand AD to include extensive scaling on all sources of damage champions now exist which have have purely AD scaling (I.e
). I did that from memory, because it's difficult to look that up on my laptop i'm working off of, so sorry if a few have some other scaling somewhere.
Even if you look at the older designs of champions expected to be played as purely AD (at least before the might have been reworked), they generally had one or two abilities which had zero scaling or exclusively AP scaling. (I.e.
even
followed this pattern)