1. UBI aims to streamline redistribution of wealth and remove a lot of bureaucracy around it, reducing a lot of the cost of the current system
2. Why the middle class specifically? Upper class could shoulder most of the costs (if there are) easily
>I would not be ok going to work to pay people that have decided that they are fine living on the UBI and don't want to work anymore.
Then don't work. We already have taxes for things which are way less useful for society. You could choose not to work and live on with the bare minimum. That's the freedom UBI brings. Now probably you'd do something or another out of boredom which would end up being useful to society anyway. You'd just do it for free instead of having to convince someone to pay for it.
EDIT: A better way to put it is to see UBI as an investment into people instead of as a cost. You give them what they need to live so that, in aggregate, they can innovate more and worry less about making ends meet. Notwithstanding the fact that this is something we can do, for somewhat cheap and would help a lot of people.