I actually agree with you about all of this. Calling mixed species “half-whatever,” and assuming the other half is always human, as well as privileging human/elf and human/orc combinations over other mixes has always been dumb, and I’m sure are why they got cut from the PHB. But, mixed-“species” people have long been and continue to be a part of D&D. The “pick one side’s set of stats and describe your appearance how you like” option is fine as one option for how to express mixed heritage, but it’s not great as the only option. It’s kind of crazy to me that the 2024 PHB didn’t include Tasha’s custom species rules and call them out as an option for making mixed characters. Although what I’d like even better is if they converted Xanathar’s species feats to 2024 as Origin feats, and removed the species prerequisite. So if you wanted to be a half gnome half Goliath or whatever, you could just take one as your species and take the origin feat associated with the other.
There's a reason there are specific words for, say, the mixed descendants of Spanish colonists and native Americans (Mestizo), or Dutch colonists and Indonesians (Indo), but there aren't well-known terms (or accompanying cultures) for most other specific permutations of ethnic or racial mixing. I went to high school with a woman of Tibetan and Ashkenazi descent; she is no more or less mixed race than I am, nor any more or less mixed race than Barack Obama or Eddie van Halen, but there aren't a
lot of Tibetan-Askhenazi people out there and their experiences have not coalesced into a narrative that one can build a collective identity on, only her individual experience.
By analogy, there are a lot of people of mixed elf and human parentage, or orc and human parentage, across the D&D multiverse for a variety of cultural, economic, ecologic or technological reasons, enough so that there are common beliefs, stereotypes, narratives, tropes and assumptions referring to these people; they can be placed in a group. That doesn't mean that there aren't people of mixed goblin and goliath heritage out there; but there's no collective identity for them in the same way that there are half-elves or half-orcs.
If I had my druthers, I would say that we should get a "Complete Book of Humanoids part 2" that gives us statblocks for people who descend from, at the very least, all of the races that were called "Common" in the 2014 PHB (which correlates mostly historically to the "demihuman" tag of earlier editions, all of whom appear at first glance to be mammals and hominids, as opposed to the more explicitly magical aasimar or semi-reptilian dragonborn). Port the Khoravar over as a half-elf, half-human, give us half-orc, half-human stats, come up with a better name than "Mul" for a half-dwarf, half-human, but don't stop there. Stat up a half-elf half-gnome, a half-gnome half-dwarf, etc. I'd slap down $25 for DnDbeyond DLC of punnet squares.
