... traditional astrology doesn't really care about astronomical reality, it is much more a symbolic cosmic view.
Modern astrology cares more (I think) about astronomical reality, that can be seen as an approach more materialistic (of course it depends on the point of view
). Very often modern and traditional denigrate each other (the main reason why I left astrology forums), well...
Concerning the traditional approach, for example, Venus is seen as cold and moist, whatever Venus really is from an astronomical point of view.
With regard to the 12 Signs of the Zodiac, to add a 13th sign would be a heresy!
Actually we need to consider that astrology is a perfect system in itself, with nothing to add or to remove. Concerning asteroids, personally I don't consider them at all.
Well considering that it was the Traditional Astrologers that first categorised and established the constellations and what we now call Astronomy, I think it's rather a mistake to say that they didn't care about astronomical 'reality' they actually established it in the first place.
However, more importantly, why do you consider Ophiuchus (or for that matter, any constellation) to be 'real'? I'm assuming here that as you mention asteroids and the outer planets, you think Ophiuchus has the same property of reality as they do, that it it has objective existence and can be measured. Modern Astronomy would certainly not see Ophiuchus or the constellations as real entities (as opposed to the stars that comprise them). They are at the most, convenient (and arbitrary) boundaries for identifying regions of space. They also are a Western concept.
Ophiuchus is mentioned by Manilius in his first century AD poem,
Astronomica and was clearly known and categorised before that. But unlike many modern Astrologer, Manilius knew the difference between a sign and a constellation and clearly says so. Ophiuchus is a constellation. Since the International Astronomical Union of 1922 Astronomers have treated it as being a separate constellation (not sign) which intersects with the ecliptic. The IAU decision is one of categorisation, not in some way, altering our perception of reality.
Signs always were different from constellations. They used to be seen (and still are in Astrology) as the backdrop against which the constellations are seen. They too, are arbitrary divisions, beginning in modern Western Astrology, with the March equinox and proceeding in thirty degree segments through the 360 degrees of the circle. The took their names from the principle constellations that fell within them, and still do in Vedic Astrology.
The precession of the equinoxes has altered the alignments of constellations and signs in Western Astrology. The advantage of the Western model is that it keeps signs in keeping with seasons. The disadvantage is that it shifts the alignment of signs with the constellations that gave them their names.
If you read the texts on Astrology between Manilius and the late Medieval Period, you will find an awful lot of what we would now call Astronomy. Look at a modern text and you will find virtually zero Astronomy apart from a definition of the Zodiac, and an explanation of Ascendant and Midheaven (if you are lucky). The constellations are now virtually ignored, probably because few are aware of their difference from signs, as evidenced by talk of a thirteenth sign.
Now we can't prevent Astrologers acting out of ignorance of Astronomy or their own history. It's happened before and no doubt it will happen again. Much of political and social change (either for good or bad) has followed decisions taken in ignorance. The best we can do is ask that such actions are preferably based on some form of rational consideration and that that is transferred into a rationale which provides a theoretical base for the change in approach and indicates why we should expect better readings if the change is implemented. Such a rationale should also explain why we should use a sign based approach to this issue rather than the more traditional star based approach. Remember, that Traditional Astrology referenced the fixed stars as well as the 'wandering stars'.