It does make it much more expensive, and something that people with other options won't want to be involved in. If 'dark room' trading was illegal, Goldman, etc. would exit those markets.
It's true that people break laws, but the extreme idea that laws and law enforcement are therefore useless is obviously wrong. Look, for example, at the Republicans and their allies banning DEI - it's had quite an effect.
Millions of people will be doing it, irregardless if it’s banned or not.
The best thing you can do is let it happen, monitor it and call out bad actors.