"Why bother investing in a huge corp network when you can't trust it anyway?"
Redundency in security.
And its in-hand. You can fix it, expand it, modernize it, control it, instead of shifting all that responsibility to some third party to which you are merely another customer.
Trusting nothing, protection at machine level, the user level, the application level and the data level will not do away with the corporate networks.
If anything, it may have the opposite effect, and encourage more use of such wholly-owned networks, perhaps melded with some cloud services.
But as sooner we move away from the Maginot Line mentality for our networks the better.
It may seem counter intuitive in the physical world, but a point defense system is easier to implement in computer networks than in the real world. Each computer should protect itself. Build this in from the beginning and it just happens naturally each computer, each file, each application. Because relying on the stockade to keep out the attackers hasn't actually worked that well in the physical world, and costs a boatload of money and expertise in the network world.
What good is ipv6 if we all have to hide behind firewalls forever.?