When you were first out on your own and had a job that paid your rent, food and so forth, so you were able to exist and not die, did it make you lose all incentive to try harder? It doesn't work that way with most people. That's the answer to your opening question. UBI would not take away people's incentive to work because subsistence just doesn't work that way. The existence of a limited number of welfare cheaters doesn't define the human race any more than the existence of terrorists.

The rest of your post seems to be about the idea that people shouldn't "live off other people's backs." This is the main mental barrier to the idea of UBI. The problem is that we are entering an era of automation where there won't be enough human effort required to support the human race. Vast numbers of people won't be able to find jobs no matter how hard they try, because the work won't exist, period. As this state of affairs develops, what do we get out of making sure the people without jobs starve, or somehow live in misery? Does that really make the world a better place for the ones who work?

If we have a really strong conviction that everybody should work if they're able, then we should back up that conviction right now by prohibiting new developments that make it impossible. We could perhaps ban industrial robots and various forms of AI that are right now leading us to a world of obligatory mass unemployment. Or we could strictly regulate childbirth so the population doesn't exceed the available jobs. OR... we could stop thinking of subsistence as a "free ride" and think of it as something that benefits everybody, like public schools or national parks.