I'm probably an outlier, but as someone working a non-physical job, I have been talking about how I personally would still keep working and doing jobs even if all of my basic needs are met.
I do work purely because of idealistic reasons. I'm not actually owed anything, I'm working because of us, humanity.
Yeah, people are going to slack, and that's just life. It's just statistics, optimize society to such a configuration that people acts have a place and get the support they need. Most of the "slackers" who represent the people just living on the UBI type system you described, probably don't do it as a direct choice. Most likely they feel like they can't contribute to society, but that's something that's solvable. It's a matter of education and culture.
About your quote:
"have nothing to lose and decide to attack the source of their misery. Bread and circuses will only distract so much."
There's a theory directly relating to this that I have about the modern golden age of punk and rock in the nordics.
When people had all of their basic needs met, they finally had the time to take a breath, take a look inwards and face their own inner demons. I suggest that this was one of the big things that created the conditions which made the nordics the rock/punk powerhouse that they were.
Of course, the good times also had their downsides, lots of complacency in politics and not feeling the need to fight for better, because things were already better than they've ever been!