The first problem is that a DeX is actually an unsolved problem. There are no, true, trustless, decentralised exchanges in existance, because blockchains cannot support anything more than their own tokens, and there is no standard atomic-swap protocol.
I suspect you are right and I have looked at creating a truly decentralised node discovery based on a billion user and using
the public key of nodes to sub-domain the name server nodes but it basically comes down to 65535 encrypted dat files
but even this won't scale because the files can still become too big and slow to load.
Trying to keep sub-domain servers in sync with each other will generate a huge amount of network chatter
so yes it's possible to scale to 50 million accounts using my code but it won't be close to real time without big centralized
servers and without a data center the size of Googles using my approach it could never cope with a billion users.
Throw in that most of the nodes won't have open firewall ports and they are only accessible via relay nodes and you start
to see the size of the problem.