You seem to be arguing that talking about races within humans may be useful even if the reality only approximates the definition of race (similarly to the idea of a "circle", which even though it does not apply in all it's precision to any real object it may still be a useful concept as an approximation nonetheless). However, I don't think that comparison is particularly insightful, and it may even be a bit misleading in my opinion because of the important differences in how those two things are defined (circle and race).

After all, the reason why no real object is an actual circle is because the definition of circle is so to say an "ideal" definition that no real object can fit in all it's precision. It's natural to assume that no real object will have all of it's "points" perfectly distributed according to a circle's equation (without even getting philosophical as to how these mathematical definitions relate to the real world, or if they do at all). If one rejects any "approximate", non exact application of the concept, then it will be mostly useless when it comes to describing or understanding the real world (because you won't be able to use it for anything).

On the other hand, the concept of "race" is quite the opposite to ideal: it's not "ideal" as the circle is, in fact it's more of a pragmatic/working definition. It's more like the definition of "chair": many things may or may not be considered a chair, but usually people don't feel that there's "no such thing as a chair" in the real world. On the contrary, it's more common to feel that anything "could" be a chair because it has a malleable definition based on the context, instead of nothing being "precisely" a chair because there are some rigid constraints to the definition that no real object can actually fit.

When the idea of races within the human species is pushed against, it's not because "race" is an ideal concept that no real thing may implement in all it's precision (as would be the case with the circle). I won't present these actual reasons (which could get quite political) here, but I will say that I definitely wouldn't consider those two claims to be in the same category:

- Saying that X real object is not a circle, or that no real object can be (exactly) a circle has to do with the fact that the concept of circle is ideal and by definition nothing "real" will fit it perfectly.

- Saying that (in the human species) there are no races is, however, not based on a quality of the definition of the concept of "race" (specifically, it's not ideal), but on some quantitative judgements about what kind of thing qualifies as a race an what doesn't (pretty much like the concept of "chair", "food", etc. which are not ideal and there's some room for discussion based on context when it comes to whether some specific object fits the category or not).

Ok, so saying race doesn't exist is like saying chairs doesn't exist, since you can't really say what is a chair, what is a shelf and what is a table, correct? Technically you could say that a chair is a table or a shelf, but people still like to call them chairs, you know the difference when you see it.

Races is like that, scientists can't define it but its still a useful concept like a chair. Scientists can't exactly define what a chair is either, but its still a very useful concept and we can discuss chairs and everyone understand what we mean.