Privacy coins may be good and all for everyday people. But if they're heavily used in the "Shadow Economy", they could face constant opposition from governments worldwide. This would lead to a full ban on privacy-oriented coins like Monero, Grin, Beam, Zcash and the likes. While Fiat is commonly used for illegal activities (instead of crypto), we cannot discard the possibility of privacy coins being used massively in the "Shadow Economy" sometime in the future. For that to happen, crypto prices must be stable on the market. Otherwise, there will be no use for a crypto-oriented "Shadow Economy".

As long as privacy coins are used legally, the government won't crackdown on it anytime soon. One would expect governments to declare privacy coins "illegal" if they were used at a large magnitude by drug dealers, money launderers, and other criminals. Even now where privacy coins are barely used for illegal activities, governments are being skeptical about them. We all know that the government wants to have power over one's capital. Considering that some privacy coins are completely fungible, it makes government's efforts to spy on you totally worthless. This will be a never-ending battle, as privacy coins try to find their way in the mainstream world. Of course, there are people like you and me which uses privacy coins legally for preserving privacy against prying eyes. But the government doesn't care as long as it gets what it wants (which is the ability to track & trace transactions for taxation and other purposes). The "Shadow Economy" will continue to use Fiat (physical cash) as it's stable while being anonymous at the same time. Just my thoughts