We still have tropes. The pop culture notion of what the different fantasy races are like goes well beyond the current edition of D&D. Letting players swap out racial ability score increases won’t suddenly make people stop thinking of dwarves a short and and stout, elves as lithe and nimble, halflings as small and clever, or orcs as big and strong. The type still exists to play against, it’s just that now, if your DM chooses to allow this optional rule, you’ll be able to play against it without being less effective at your class role than someone who played to type.
Perhaps somewhat ironically, I’m actually not the biggest fan of this change and probably won’t be using it at my table. I’m a strong advocate of not shoehorning certain races into certain classes via racial ASIs, so you’d think I’d be all for this rule, but to me it feels lazy. Instead of actually designing races that feel and play truly differently without the crutch of ASIs, they just let you swap out features however you like, effectively making race a purely cosmetic choice. That’s... kind of the opposite of why I am against racial ASIs.
Of course, I recognize that the racial redesign I would want is beyond the scope of Tasha’s Cauldron. It would require a full on 4.5 or 6e (or A5E...) to really pull off in a way I would find satisfying. As it stands, this optional rule is a fine band-aid fix for groups who want that kind of flexibility, and I’ll continue doing what I’m doing on my own.