At this point it feels like Mach is a reliable source of bugs in macOS. I know Apple is working hard to lock it all down, but is there any path to shifting away from Mach completely?
To be honest, any sort of locked down messaging system requires more (i.e. validation of sender, etc.) than just the transfer of messages. And that's just not something you would get with a low-level communication protocol like Mach (unless Apple overhauled the MIG compiler to add entitlement checks).
Mach is fantastic when paired with entitlement checks.
Technically, this might actually increase the potential attack surface (due to more different components existing together). But more specialised surface would then allow for simplified control and as an effect better protection of that surface.