Vision differences are not off the table. People claiming that humans can have Darkvision in Tasha’s are misrepresenting the optional rules.
OK.
There are plenty of other differences that are on the table. Descriptive differences of course.
Which is fine, but fluff; I'm after mechanical differences.
Mechanical differences such as inherent spellcasting,
OK, that's one.
That's pure fluff unless that species gets a Strength bonus, which is off the table.
abilities like the dragonborn’s breath weapon or the Dwarf’s poison resistance, the halfling’s ability to pass through the spaces of larger creatures...
That's two, three and four - so there's still some - however...
anything and everything beyond inherent alignments, ability score adjustments, and racial proficiencies.
From here it looks like the Dwarf's poison resistance and the Hobbit's ability to pass through occupied spaces are simply 'racial proficiencies' using another name.
None of these, however, are as basic and-or intrinsic to the species as are stats. I've no problem whatsoever with each species
within itself running on a 3-18 bell curve for each stat: Hobbits have their own 3-18 bell curve for Strength, as do Humans and Elves and Minotaurs. Cool.
If parties were always made up of a single species no adjusts would ever be needed.
But, the believability problem arises when you take those disparate 3-18 curves and try to compare them against each other in a mixed-species situation e.g. the typical adventuring party. Humans are the baseline and always have been, so the question becomes how does an 18 Strength (or any Str score) on the Hobbit curve map to the Human curve. Ditto for Elves, ditto for Minotaurs, and on we go.
To suggest that all these map out the same - that Str 18 on the Hobbit curve is the same as Str 18 on the Minotaur curve - is kinda ludicrous. And so, we get species-based bonuses and penalties to stats to reflect how they map on to that Human 3-18 curve. For the physical stats - Str, Dex and Con - I just can't see why there'd ever be any issue with this.
The mental stats - Int, Wis and Cha - might be a different matter; and maybe there all the 3-18 curves would map pretty closely to each other with the exception of the physical apprarance aspect of Cha: some species are simply more attractive overall than others.