If only it were that easy. So much of security is a case of people abusing behavior of a complex system. Its difficult to image how some of these complex interactions might be exploited ahead of time.
This is a case where for the most part the system is working as designed. A high amount of traffic is detected so the system pushes the devices to fall back on legacy resources so the system of call handling over all can continue to function. It just so happens the high traffic isn't a bunch of devices all wanting voice and data at one but a basically a DOS attack. There isn't much you can do about DOS attacks on over the air media. If someone wants to jam a signal they can. This is basically that but they happen to be doing some protocol participation as well.
Availability is part of security, should the system just fall over under high load instead? Would that be 'more secure' in your estimation or less?
Honestly what should probably happen is the handset should ask.. "LTE fallback requested, voice and data privacy not assured, do you wish to proceed?" Now you are back to a human problem where they have to make a judgement call. They also have to be savvy about the situation, and ask themselves is this likely happening because of some congestion event or is this an attempt to MITM me?