I bet if that scenario where exchanges are banned there will be surge of P2P platform appear and people gonna just do what they're doing back then when we're still on the early bitcoin days.
Just do transaction with other peer and build reputation system.
These exchanges are 100% replaceable in that scenario where they are suddenly dysfunctional.
These centralised exchanges are just killing the original concept of Bitcoin i.e. p2p electronic cash system. Though the entry of big institutes into Bitcoin has a positive impact on it's price they have altogether eliminated the user privacy. If you are going to centralised exchange then you are doing compromise on your privacy.
These centralised exchanges came into existence way after Bitcoin creation. In fact they are created to gain profit from popularity of growing Bitcoin business. If they cease to exist today, Bitcoin will continue it's normal operation as it was prior to creation of centralised exchnages.