How can they assume this will not change how everything else works? Yes. You can say statistically we did the comparison by holding everything else the same, but it only means "statistically". It definitely does not mean in reality you can hold everything else the same. There is no way when the policy changes, the system still works as before. This kind of study is totally delusional. A policy works in other countrie…
On the other hand, how else are you going to do it? Do you know what the second-order effects are going to be? How sure are you? Can you prove it to someone who doesn't hold the same intellectual framework or political leaning that you do?