Besides security and customer support to get private keys back for users, DEXes should have passive income for their customers, whom own exchanges' coins and use them to support DEXes. This approach, like some of DEXes applied, will help to increase total liquidity significantly. However, such thing on liquidity can not be reached in short time, because users have to be convinced that DEXes they use are reliable enough to store their coins directly on DEXes platforms, in order to get passive income. It is always risky to store coins on exchanges, and higher risks if they are DEXes.