But on the point about mechanics matter.
To me, species should largely go the same route as alignment. After all, while alignment has zero mechanical impact in 5e, it does have a significant impact on play. Someone playing a LG fighter is going to play that character differently than a CE fighter, despite alignment having no actual mechanics.
And the trajectory for species does seem to be following alignment. In early D&D, alignment had HUGE amounts of impact. Your choice of alignment affected what classes you could play, how spells interacted with you and gave your DM a honking big lever into controlling the behavior of your character (the rules for changing alignment were punishing). Then, later down the line, alignment becomes far less restrictive. Most classes remove alignment restrictions, changing alignment has little impact on your character and spells and magical effects key less and less off of alignment.
Now we have 5e. Alignment has zero mechanical weight. None. There is virtually nothing in the game that keys off alignment. Yet it still has an impact on play. We talk about demons and devils being evil. The party is typically presumed to be good aligned - at least the core books assume that. And players do take alignment into consideration when playing their characters.
I would argue that species is very similar. The mechanical impact of species is very minor. Based solely on mechanics, no descriptions, nothing else, it would be very hard to tell an elf from a human from a half elf. After all, cantrips are hardly species specific. Lots of classes have cantrips. A human wizard, an elf wizard and a half-elf wizard, stripped of flavor, only using mechanics, would be pretty hard to distinguish from each other. Not impossible, that's true. But, not easy.
But flavor wise? VERY different. They should be. They should play and look very different at the table. And, typically, they do (although I've seen an awful lot of "humans that see in the dark elves" which always grinds my gears). Not because of the mechanics, but, like alignment, because of the flavor surrounding the species.