It is always going to be a field for rules optimization. That's the point I'm trying to get at.
Trying to reign in optimizers is a fool's errand. Even with the proposed "pick-a-parent" system, optimizers will identify which species statblock is "superior" for a given optimization purpose and make that the default. The only difference is that they won't have reason to make their characters "half-elves" or "half-orcs" (or any other mixed heritage) anymore, because functionally those options no longer exist as mechanical choices to optimize.
The only thing accomplished by making mixed ancestry purely a roleplaying choice is discouraging anyone who wants their character's ancestry to be expressed in their game mechanics from bothering to play mixed ancestry characters at all. Anyone who wants their choice of mixed ancestry to be reflected in their mechanics now has to which fraction of their heritage is the one that the game recognizes as legitimate, and/or spend additional character creation resources (background, skills, feat, etc.) trying to make up the difference.
Your character either gets less of their heritage in their mechanics or it costs them more to get "all" of it, which makes mixed ancestry more "expensive" to play and is thus an incentive to simply play a single ancestry character from the start.