The leeches have to be careful, however. Their survival and well-being depend on the cow remaining alive. If the leeches get too greedy and too voracious, they will suck so much blood out of the cow that the cow ends up dying, which is not a good thing for the leeches. 

Thus, the leeches must work out a balance in which they suck just enough blood out of the cow to keep the leeches happy and prosperous but not so much blood that the cow dies.

That's what statist politicians and bureaucrats do with the welfare state. They aim to suck just enough money out of the producing sector through taxation to keep them well and prosperous, but they take great care to not take so much booty that they end up killing the productive sector.

But what happens when the cow decides to just move away? What do the parasites do then?

That's precisely what has been happening in the state of California, one of the biggest welfare-state havens in the United States. After several decades of plundering and looting the producing sector to fund its welfare-state way of life, the producers have been moving out of the state to protect themselves from the parasites.