I'd be fine with "Mix and Match" being relegated to the DMG too - indeed, I've proposed that before. I just want some indication that there will be core system options above and beyond "Pick a Parent" that don't leave mechanically minded players like myself hanging in the wind on this or shove the responsibility of properly addressing it onto 3rd parties and homebrew.
Species mechanics are a small but important part of how I conceptualize who a character is, and not having some kind of distinct mixed ancestry traits to work with (even if I have to piece them together myself) renders the whole exercise of creating a mixed ancestry character shallow, uninspiring, and meaningless. A Khoravar using human traits doesn't feel like a Khoravar to me, it feels like a human wearing fake, pointy costume ears.
And again, if "Half-elves/orcs shouldn't get special treatment by being the only mixed options with unique stats" means that no mixed options can get unique stats, then what happens the next time they take a look at Eberron? Khoravar and Jhor'guntaal aren't the only mixed ancestry populations in the setting, but they are by far the most prominent. If the goal is less about actually supporting diverse mixed ancestry options and more about making sure half-elf/orcs don't stand out from the crowd, then that doesn't end well for Eberron's existing lore, and this is a setting that very specifically doesn't do the "half-orcs are primarily the result of nonconsensual boinking" stuff.