This is wrong. The Cherokee hack existed because both CAN buses in question travel to the infotainment system. The vulnerability came into being by finding a path between them. It became remotely exploitable when they found a way to do the same thing over the onboard cellular modem. So no, your definitive crude architecture defect and reference to the 1990s is a misunderstanding of the actual problem.

This is a common FCA design. CAN HI and CAN LO often go to the same device on different pins. My Jeep has an instrumentation bus as well and all three arrive in one box in multiple places within the vehicle. Friend with a Rubicon Wrangler discovered both engine buses connect to the electric motor to disconnect the sway bar. He discovered this when water got in it, shorted it out, caught the electric motor on fire while he was watching, and subsequently totaled the ECU and TIPM by transmitting electrical noise on both CAN ECU buses.

You’ve assumed one bus goes to one thing to reach your conclusion. That is faulty logic, which is no shortage of ironic given the parts of your comment I’m mostly ignoring in this reply.