While it's true that the attack vector you describes exist, assuming you can trust both the driver and hardware, it is hard to exploit as most software cannot t

While it's true that the attack vector you describes exist, assuming you can trust both the driver and hardware, it is hard to exploit as most software cannot talk directly to the driver. Even if it does, manipulating it the way you describe is challenging. By comparison, it's not only easy to talk with most drivers in a monolithic kernel, there is also many more ways to manipulate it which will result in the entire system being owned. Security isn't all or nothing if designed robustly. I hope the case for IOMMUs becomes stronger due to increased popularity of Operating Systems with userspace drivers. The BOM cost is simply worth it.