Maybe a new analogy helps.

SW presents a blended price and blended basket of two goods. You can interact with the Service through the blended price, but that does not erase the fact that the basket contains two separate from similar resources.

A different set of economic actors uses one resource, and/or both. There are explicit incentives to shift actors from solely using one resource to using both.

The fact that separate sets of economic actors and incentives exist is sufficient to prove it is indeed a basket of goods, not a single good.

On Wed, Dec 16, 2015 at 4:36 PM, Pieter Wuille <pieter.wuille@gmail.com> wrote:

Thus, the miners' best strategy is to accept the witness transactions,
as it allows 1000000/110=9090 transactions rather than
1000000/200=5000.

Under your blended algorithm, this seems reasonable as a first pass.

In fact, the optimal fee maximizing strategy is always to maximize fee
per virtual size.

This is a microscopic, not macroscopic analysis. Externalities and long term incentives can severely perturb or invalidate that line of thinking.

Typical counter-example: Many miners are perfectly happy with very low fees to encourage long term growth of their bitcoin income through network effect growth -- rendering fee micro-optimizations largely in the realm of DoS prevention rather than miner incentive.