Bitcoin mixers and coin mixers in general are only designed for criminals and are only used for criminals.
The argument actually goes the other way round:
mixing in general is mainly a necessity because e.g. exchanges are complicit in the criminal act of "chainalysis" which is pretty obviously contrary to the privacy rights of e.g. European citizens.
Everyone who tries to find an contra argument will be sad because their aren't any
Sorry, I've proven you wrong.