Uh. That's a lot.

Morality is probably the biggest thing with slavery. But besides that if you have to many slaves you get a system vulnerable to being overthrown. Also I don't think it actually works out more productive. I seem to recall reading that northern farms with hired hands actually had higher net profit than southern slave farms.

Also I do believe in our current system food, shelter, water, and clothing are already provided to all. So the cart isn't before the horse with giving stuff away for free.