There was "post-scarcity for basic survival needs" for millions of years already. When life first formed on earth, there was food enough for every organism. That didn't prevent organisms from fighting each other.
Competition and the extinction of the losers has been a central theme of life since it began. Being at the top of the food pyramid, and in a first-world country has made us forget that fact. The idea that humanity can rise above this, formed from living in this social-economic bubble for far too long, is just hubris.
That's where the big difference comes from your example of '"post-scarcity for basic survival needs" for millions of years already.' there's never been the coordination and impartiality to distribute and coordinate at that level.
As for today if we coordinated and there were no economic considerations we could feed everyone today. It's just that there's not the planning or monetary forces in place to make it happen.
The future will hold a very different scenario than a typical first-world country today where the populous actually produces useful work. As a leader, instead you have a population who who in-fight amongst themselves for the best of the consumable goods and produce absolutely nothing of value for you. To make matters worse, they continually attempt to push their boundaries, growing bigger and bigger if not thwarted (as life normally does).
My other point, that techno-baron A will want to fight/defend himself against techno-baron B still applies. Won't anything that techno-baron A needs to do to defend himself against techno-baron B trump the humanitarian concerns for maintaining the lifestyle of his subjects that give absolutely nothing back to him?
Even if techno-baron A is morally inclined to do so, there will be others that are more brutal and would not. And even if the moral techo-baron A is able to completely conquer the world (seems very unlikely considering how schizophrenically both brutal and compassionate he would have to be), then his hold on it will at some time cease, probably under duress from threat(s) within.
Why would people follow it? Why would some rich and powerful guy with connections forgo all his power to some CPAI that was set up? How would he maintain his ego when the world no longer shakes from his every footstep? How would he repay all the favors he made to those that got him there?
AI is just a tool, like the gun, the ship or the railroad. It's not some panacea that is going to protect human nature from itself.
The idea is that in a post singularity[1] world we won't be making those decisions. A hopefully wise AI will manage and decide everything for us. (Even if we wouldn't want it to.)
That or the AI decides to exterminate us. In both cases jobs won't be a major concern.