What if the disruptor is some sort of LLM service like ChatGPT and related AIs/robots that do other physical jobs? That potentially blunts the impact of low birth rates.
Not at all, society literally breaks down if that is the case because those services would displace 2/3 of the work out there.
Hungry people riot, unrest ensues, and most people have been taught (as a core identity) that if you work hard you will be better off and have a good future for yourself and your children. Those core identity promises being recognized as lies would drive a mass movement of mostly irrational people. You then get the law of reverse effect (psychology) driving this. Who is responsible, who is to blame.
Someone will always have that answer and it need not be true because at that point they won't care.
Also, at 2/3 the population, you are also dealing with intelligent irrationality, a misnomer but the potential risks from that group are much higher, complex, and more costly.
Intelligence is speed and planning.