What is a native script, though? If you mean the one that is completely original, then very few languages could claim such a thing - most cultures borrowed their writing systems from someone else, and then adapted them to their needs. Most people would say that Cyrillic, for example, is a "native script" for East Slavic languages, but it's clearly derived from Greek (and its historical version more so than the modern one).

For Kazakh and their predecessors, the script in use before Arabic was Orkhon. But it was also derived from other scripts.

I agree, like many things, it's a continuum rather than a binary, but if we are going to draw a line somewhere, my heuristic would be when the script is named after a foreign language, it isn't native. So, for example, the "Arabic," "Chinese" and "Latin" scripts are native, respectively, to Arabic, Chinese dialects and Romance languages, while in Farsi, Japanese and English they are in customary use, but are not of native origin.