I think nothing can be done anymore, but I don't buy that security through obscurity never worked in practice. A better way to look at this is effort rather than obscurity. The effort it takes to do something with AI is going down, not up.

Almost everything was possible given you put in the effort, but few people possess the will to put in the effort AND pursue a malicious goal.

I think an obvious example is all the fake ai content flooding the internet made to trick people in exchange for money (ad revenue, scams, likes, etc). This existed before ai, but I think it's fair to say pumping out content now requires less effort than it did before.

Most physical locks are an example of security through obscurity/effort. You can after all just pick a lock if you go through the effort to learn the skill. But once a universal lock picker is made available to everyone, you will simply see more locks getting picked.