Drivers are the commercial case for Fuschia. But in general, microkernels make it much easier to 1) implement privilege isolation for subsystems and 2) implement subsystems in a more secure manner, both of which absolutely improve security posture. A subsystem is just another type of driver. Though, it depends on how well Zircon makes use of this--i.e. avoids implementing all the most critical subsystems in the same process, or otherwise abuses too much unprotected memory sharing among them.