There's a natural equilibrium between the amount of welfare your state provides and how porous your borders can be.
Open borders only make sense with no welfare state, so opening your border will inevitably lead to the collapse of the welfare state.
Discussion
As I said, elsewhere, I'm okay with the welfare state collapsing (it will, anyway, eventually), but
I think most of the people promoting open borders don't realize that it will result in all welfare being privatized and largely disappearing.
Well, I am being charitable.
Some of them are probably promoting open borders because they know it'll collapse the welfare state rapidly and eventually lead to anarchy.
A lot of people are eager to build on some ashes, so they're arsonists.
With a well designed welfare system, the welfare state will eventually die. It's a knowledge based economy that'll create the critical thinking that'll outlive the use case of welfare.