Re: Most of the things people regard as threats are what hope there exist for the future.
Which really only leaves three routes I can see that a society could go down, assuming it reaches a post-scarcity state:
1. Mass unemployment and poverty. The idle and desperate poor would create a population ripe for rebellion, so a place state would be created to keep them in check. The scenario of Manna. The unwashed masses would be swept off to some out-of-the-way place to serve as a source of cheap labor for those few jobs that resisted automation, and kept pacified by tried-and-tested social manipulation techniques. Although resources would exist sufficient to transition to state three, there would be no means by which the masses could access them, and no incentive for the ruling class to compromise their own wealth and power. Manna's comparison is apt: Those with money to spare wouldn't wish to support a welfare state for the same reason that all of us can be aware of the poverty in the third world yet feel no great desire to donate more than a token sum to charities to help.
2. As above, but the police state fails: Violent revolution results, triggering a transition to state one or three. The situation is (rather correctly) interpreted as class warfare. Unfortunately history has shown that these popular uprisings don't tend to work out very well, and are more likely then not to end with a new totalitarian dictator in power.
3. Popular support manages to trump money. This is unlikely to happen in the US (Where 'socialist' is a dirty word) but is a potential route in much of Europe. Basic income established. Much fuss is made by the rich who find a substantial chunk of their wealth 'stolen' by the government, but the plan goes ahead. This results in a large unemployed population who can live lives of comfortable leisure. The upper class continues to live in unimaginable wealth while complaining bitterly that thanks to the government theft they can only afford twelve bathrooms in their mansion, and they really wanted fifteen.
Three may sound like a utopia, but it's also untested: No society like it has ever existed, or could exist today. It's been tried, but the socialist ideal always fails when scaled up beyond a small community. It's possible that vastly cheaper production combined with the organizational power of computers could change this, but there are just too many unknowns to say if it is remotely viable.
All of this depends upon post-scarcity, though. It's not guaranteed to happen, as such a condition would require major advances in several areas of technology: Robotics, energy production, transport, manufacturing, recycling. It may even be counteracted by depletion of natural resources.