Fungibility Bitcoin

Can someone explain why do we need fungibility on Bitcoin? I mean if someone steals your bitcoins and they become tainted and no one accepts it, it becomes useless for thieves. It is like a kill switch inbuilt . Any crime related bitcoins can be tracked and tainted making crime impossible to perform with such tainted bitcoins.. tainted bitcoins which are not accepted by anyone is like auto coin burn making bitcoins further scarce further benefiting community. I think it is a feature and not a flaw.. Satoshi designed it that way intentionally.. it’s humanity is only way to crimeless world..