I havent assumed that he will up and leave, but you are assuming that your life will always be this perfect, and thats what I thought once too. I was smug, safe and happy.
You are assuming that life wont change, that what he says now will still be true in a year, 5 years, 10 years. That what you feel for him will be the same in a year or 5 or 10. You are assuming that should the worst happen, you will go out there and get yourself a job (thereby assuming that there is a job available for you) and assuming your parents will be able and willing to take care of your child while you do it. You are assuming good health on your and his part, and on the part of your child.
Life happens while we are not looking, and one day you may well wake up and find that the worst has happened in one way or another. You may find yourself a single mother, sacked, ill, disabled, being a carer to your parents, your husband, your child. All of these things happen to people throughout the world, every single day.
My point is that you just dont know and to say that no one should be allowed to rely on the welfare state just because, right now, YOU dont need it is very short sighted.