Yes, to be fair that's a reasonable point. Astrology is indeed a living craft with a history that dates back 3000 years. There have been many innovations in that time, some of which are still with us, such as the system of signs and sign rulership and others which have largely fell by the wayside, such as the large majority of the Arabic parts. My point is that such innovations should be seen as a natural development of the tradition rather than an arbitrary bolt on.
The original seven planets of Astrology have been used since prehistoric times, the myths which they symbolise are just as likely to have developed from observing their nature and behaviour than being imposed on them.
In the case of the outer planets, the naming of these planets has been done by Astronomers, with obviously no Astrological basis (as they rejected Astrology). The Pluto mythology may or may not be appropriate for this planetoid but it seems to me that modern Astrologers have tried to alter the nature of Scorpio to make it fit their idea of what they thinkPluto symbolises. And all this before Pluto has completed even half an orbit around the Sun. Indeed they seem to have some difficulty in separating the meaning of sign from planaet and indeed from house, if the prevalence of the Astrological alphabet is anything to go by.
If Pluto is to be used, then you should have a good reason for using it - it is dangerous to simply accept it as having Astrological significance simply because the text book says so.