> People feel a natural obligation to be rewarded for their work ...
If people are actively demotivated by seeing others idle and surviving, who cares? Provided that we as a society can afford basic income (including after it's been established and it's having various impacts on the workforce), it's no loss if people stop working. And there's only so much basic income could ever give you; it'll make the necessities of life free (or rather paid for), but if you want things beyond necessities (including any positional good), you'd still want to look for work.
> Is a safety net nice? ...
If we can afford to provide a basic income, you don't need to work -- unless the economy goes so pear-shaped that we stop being able to afford a basic income. (And this seems unlikely, since automated factories won't be offered a basic income. If anything, basic income might increase consumer demand...)
> The idea of giving people a flat income ...
I'd emphasize that this income will grow if you work -- and that it won't be enough to buy positional goods, by definition. You can keep body and soul together on a basic income, but to live better you'll have to find a way to engage with the economy. (And the presence of a hammock-grade safety net will give you more options, not fewer. Live on oatmeal and vegetables, and buy stock with your food savings; or work for years on a project that might or might not pay off.)
> I'm not sure what you mean by this ...
"Made to do work" could mean any number of things. Tribal warfare, hunting, gathering, and social climbing are certainly things humanity has evolved to do; subsistence farming, artisanal crafts, and management are close enough that we're pretty good at them; but factory work, we're just plain not adapted to. Keeping strict schedules, doing small parts of a large whole, and repeating a single task for a long period of time: these are learned skills at best for humanity, and that's why such jobs get automated.
> Some people feel like they wouldn't be fulfilling their purpose ...
They can find jobs and keep working. UBI isn't the end of employment, just the provision of support. Think of it as reverting to the economic structure of the classical world or the American South, except that this time the only slaves are robots.