Trusting your private network is private turns that whole network into a candy store once a beach head in that network has been established.
Defense in depth exists for a reason.
Defense in depth exists for a reason.
For most people, in most practical scenarios, you have to hang your hat on something.
Yes, take basic precautions, but if you lacked the chops to keep your private network private, then you have little or no chance of preventing the ensuing attacks.