If the microphone is built into the laptop it should have a hardware switch. That still has problems as you note, but it is far better than the current state of

If the microphone is built into the laptop it should have a hardware switch. That still has problems as you note, but it is far better than the current state of affairs for most devices. And it still requires trust in someone (or opening the device yourself). A better solution would be to have the microphone be physically removable, but even then it could have an internal power source and storage...