Your conclusion is only partially right. And your have the wrong root cause. You think the engineering approach being used is the right one but the effort was underestimated. That is incorrect.
The process being used by most AV makers to develop these systems, public shadow and safety driving, is untenable, has killed six people to date for no reason and will kill thousands more when accident scenarios are learned. It is impossible to drive the one trillion miles or spend over $300B to stumble and restumble on all the scenarios necessary to complete the effort. In addition the process harms people for no reason. This occurs two ways. The first is through handover or fall back. A process that cannot be made safe for most complex scenarios, by any monitoring and notification system, because they cannot provide the time to regain proper situational awareness and do the right thing the right way, especially in time critical scenarios. The other dangerous area is training the systems to handle accident scenarios. In order do that AV makers will have to run thousands of accident scenarios thousands of times. that will cause thousands of injuries and deaths. The solution is to switch 99.9% of this to aerospace/DoD simulation technology and systems/safety engineering. All informed and validated by real-world data. (Not gaming engine based systems as they have significant real-time and model fidelity flaws in complex scenarios).