We're talking about a device that is completely unsupported by its manufacturer. You already have no expectation of security or integrity when the last available OS and security update is over 5 years old by now.
Opening up access to install your own software would have great potential to increase the security of the device, not least because you can actually run current and patched versions of any software.
If you have physical access to a device, you can already tap the memory bus or countless other places. Relying on a device itself to be secure when it is in your adversaries' hands is futile.
Open the hardware, use secure software and keep your storage encrypted, preferably on removable storage.
If you think that is the only way into an Apple device, and that police or even nation states are now helpless against the mighty "hack resistant" iPhone, then you may be interested in this bridge I happen to have for sale.
Opening up access to install your own software would have great potential to increase the security of the device, not least because you can actually run current and patched versions of any software.
What you're arguing for is security by obscurity.