They also don't take into account that there are more lazy scumbags than honest hard working individuals.
With too many lazy scumbags that know that no matter how little effort they put in they'll still get taken care of the whole system will collapse.
It's the old maxims of sooner or later you'll run out of other people's money or if you've got more people riding on the wagon than you do pushing/pulling it then it's not going to go anywhere.