They dont nerf because something is overused, or buff when underused (ie based on % of player use). They adjust based on ttk and weapon meta, a perfectly balanced gun meta and still everyone might use mida does not initiate a nerf.
Now, here is a question, why do so many use mida? over other scouts, over other weapon classes? this is cause for concern as it MAY indicate either a fad (ok) but it might also indicate that it is OP and need a nerf.
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They've already said, in interviews, that they've nerfed guns because they were too popular, and not just because of stats or ttk.