> Scholars use the modern Mandarin reading of the Chinese characters in question
You are telling a half truth. They are using modern Mandarin reading based on old Chinese characters that themselves were originally chosen based on how the actual name of this culture sounded at the time. In other words, you can pick any vaguely similarly sounding word in Chinese or otherwise and it'll probably be true to life enough and without the unnecessary slur.
As soon as agreement is reached on what language the Xiongnu spoke, and it happens to be a language we know enough about, it may well be that Xiongnu will be replaced by a phonetically more precise reading, just like other groups. Meanwhile, who cares.
> Meanwhile, who cares.
That's the summary and we're back to my point being that this lack of care should be embarrassing for the field.
Slavs are a extant group who you can go and talk to.