Using drastic in this instance is an appropriate, but rare, usage in a positive sense. You are correct that dramatic would be a more commonly used neutral synonym, but using drastic is technically just fine here. It is only slightly odd usage by common practice, but as a native english speaker I hardly noticed it until you pointed it out.
By using a less common word and outside of the common usage of that word, to my ear this adds emphasis, like reading the word in BOLD text or capitalized.
"drastically improved or completely eliminated"
because drastically often is a negative, improved often is a positive and increase, using a two word phrase were both words are essentially opposite of their common usage seems almost linguistic malpractice.
It would make much more sense to say either drastically decreased or dramatically decreased since the Or in the phrase indicates complete elimination.