One thing is that thought has been brout up for over a hundred years now. Thus far it has not been true. One day it might be, but hard to say when. Historically we've managed to develop ambitions to offset the reduction of needed labor of a new advance. We generally couldn't have foreseen how that was going to go down until it happened, so not knowing what that would look like doesn't mean we are at the end of the road.
The real problem is perceiving a world where we need people to work half as hard as they do today a 'threat'. That people should be able to have education, sustenance, shelter, and medicine without worry throughout their lifetime is not something to be avoided, but to be embraced. A number of people worry that people are wired such that this would be devastating to their psyche, and yet people retire all the time, go through college without fretting so much. Yes, once you get into a career you get caught up in it, but spending a good few months away can fix that.
Of course there's also the issue of fairness in a world that is 'partially' post scarcity. If you generally don't need most people to work, but desperately need some of them to, it can be tricky. To some extent reduced hours can alleviate, but some tasks don't lend themselves to such a strategy, and you can only have so-short of time frames for people to work (if you needed 5 minutes of work a week out of the average person, it'd be awkward and highly inefficient to work just for 5 minutes).