A government would still have to make and use their own keys in a man-in-the-middle attack. The forged key means that if anyone bothers to check it will be detected, and there are also various ways that an application can lock the used key to make this impossible. Man-in-the-middle requires a lot of control over the infrastructure, for something that works reliably they would need to cooperate heavily with telcos, and spend a good deal of money.