It most definitely is, they're going to deprecate the older design over time. Just look at the specs, CPU and GPU are the same between old-style and Retina, but only the Retina gets a 16GB RAM option (with 8 base, versus 4 base for old-style), a retina display, Retina gets HDMI (and a second TB, which is the future as far as Apple's concerned), an updated Magsafe design.
The old-style remains to clear inventory and to wait for the cost of making/buying retina panes to get lower.
This is a transition product just like the original Macbook Air was, which incidentally also looked like an odd duck compared to the clarity of having Macbook and Macbook Pro lines.
It's likely just a typo in your comment, but the Pro Retina is a 15.4" model, not 13".
*Not a typo, but I clarified the sentence.
I think Apple have somewhat of an aversion to having price be the only or main differentiator between products. That's why the old macbooks were scrapped. Pros were more powerful & Airs were more portable. Macbooks were only cheaper. Cheaper models can exist, but they have to have some unique advantage that'd make some users buy them regardless of price.
The existence of a Macbook pro & a Macbook pro retina bends this rule. If a 13" air and a just-as-thin 13" pro retina both existed, it would break it.