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Legend
If you keep them as a separate species representing human and elf/orc blood, you are effectively endorsing they are the only two combos who matter. That half-dwarves or gnome/halflings or dragonborn/tieflings do not exist, cannot exist, or are such freaks of nature they don't warrant mechanical expression. Even if you say that a half-elf is a blessing to the community and a half-orc a reward from Gruumsh (stripping away all negative connotations) you're still saying only they are special enough to exist.
As far as 'hybrid' options go, you are right, from a historical perspective.
For everything else, there would be Custom Lineage/Hybrid rules.