Fungibility of any coin (btc included) is only at stake if some coins are worth less than others. There is currently a hudge hype about anon crypto because of this. The story goes that if a certain coin was used in a nefarious or illegal way it is worth less since some will not accept it. THIS HAS NOT HAPPENED YET. At least not on a large scale as far as I know. Anyone please feel free to enlighten me.