dApp developers don't necessarily have to charge a fee for it to be used. Their focus is now on increasing engagement rather than revenue modeling through high transaction costs or gimmicks that would limit users to experience the Dapp.
As users use a dapp and transact within it through the use of fluid assets, a portion of the prices could go to the dev's wallet. In that instance the Dev is a partial receiver of a, say item trade, where the two parties conducting the trade earn 35% each of the reward, fluidity earns 10%, and the developer 20% (as an example).
Let's say 10k daily users, you are gaining quite a bit of money, you aren't charging your users anything extra.
In fact, you could remove fees in total which would generate high transaction volume, seeing more potential reward distribution being gained by that developer.