You can use this argument against any law ever passed. Yes there will be a black market, and yes more people will turn to it than before, but if 99% of instances of the thing you were targeting have been removed then it is still a success.
If they are not I wager chances are higher
I want my drugs legalized, but that doesn't mean I want to give them to children.
There are a bunch of other EU rules (under the DMA, DSA, GDPR etc) which de facto if not de jure only apply to very large players, but the difference is that most of the rules are either non-obvious to the user (the average user has no idea that sites want to sell their data, and if told that the GDPR restricts that will either see that as good or will not care), or obviously positive to the user (no-one is leaving threads because the cruel Digital Markets Act forces it to provide a non-logged-in browsing mode). A rule which is annoying to many users but can be evaded by just switching to an unsupervised smaller player seems unlikely to work.