My primary argument is about the Half-elf/Half-orc naming convention being inappropriate, and that I don't think they deserve special treatment over any other mixed species rules, not even due to tradition (which I think we agree has problematic origins as it is). Nor do I think that humans should be the only common denominator for mixed species, which they currently are.
And as a mixed person myself, I would ask that you kindly don't tell me I have to accept the Half-breed naming convention, or claim that it isn't a problem for a society to label a mixed people "Half-breeds". It is absolutely fine for an individual to identify that way because it is a personal choice foir how they want to be identified. But societal labeling (how it is universally presented in a rulebook for a fantasy multiverse) does not seem like the right choice.
Your argument still supports only one mechanical representation, that all Half-elves have the same racial stats. However with ability score modifiers being removed from race, mechanically they are really only just an Elf variant that is missing Trance and swapping a skill. Whoopteedoo. Mechanically it doesn't really take the human side into consideration. What if I don't want to be pigeon-holed as an Elf, mechanically?
As for my argument, Elf-kin would have more ways to be depicted. They can choose the baseline Elf assumption that exists in the current Half-elf with darkvision and elven ancestry, but if I want my Elf-kin to instead get the human abilities of my human parent, I would have that choice, and that choice is AWESOME. I would still be an Elf-kin, though. I can still choose how my character appears cosmetically, what their background is, and how they fit into their society, whether Elf, Human, Multicultural, or foreign to their preferences.
All that said, the Half-elf option as its own species entry isn't being taken away or erased, just like the Enchanter isn't being taken away or erased. The options still exist in the game. They just wouldn't be in this particular book and there would just be more variant ways to depict more hybrid species that wouldn't otherwise be an option.
You are also not being erased if a rulebook offers a variant way to create hybrid characters. The way you prefer will always be visible and available. You would still have the books and/or DDB where they are available.