My take on this, is before they will be phased out if ever they will, it will take long before such scenario will happen.
There are few jurisdictions that may be strict towards these privacy coins.
However, if the crypto exchange is registered in a country or place where the regulations in crypto are not that strict, they can still enjoy their presence.
There are purposes why a lot of crypto users like the use of privacy coins, and I don't think these coins will be gone very soon.
They existed for so long already, and they may find a way how to go around these government regulations.
The combination of DEX and privacy coins will make them hard to combat. I think regulators are going to have to do their best to get onboard with crypto sooner than later if they want to steer the ship in certain ways.