1. Under a welfare system, whether you work or do nothing has no bearing on whether you can stay alive, therefore fewer people will work. As fewer people work, more taxes are needed to support the increased number of people on welfare. These heavier taxes mean less income for those who work, hence they further diminish the monetary difference between working and not working, which causes yet more people to quit work, ad infinitum.

this ignores that people in the world have initiative and want more than mediocrity. the so-called "slippery slope" which you talk about is usually more like a "sticky step."

This situation couldn't continue ad infinitum because a workforce is necessary to maintain a welfare system.

2. In the case of old age pensions, the welfare system (by providing "free" money) effectively encourages people to stop working as early as possible, and makes less serious the problem of caring for your parents in their old age. This means that relations between the generations tend to deteriorate, since it's easier to ignore family ties if you think that they'll never be necessary for anyone's survival.

in the real world, retirement ages are created.

3. The welfare system provides more money to single parents than to wedded or coupled parents, thus subsidizing the creation of single-parent homes.

this may be true, but few people shape their personal lives based on receiving minimal government benefits.

4. By giving money to those with incapacitating physical or mental illness, welfare encourages people to fabricate illnesses and exaggerate the seriousness of existing illnesses. This puts a strain on the medical system, which necessitates higher taxes, which lead to the effects noted in (1).

again, this assumes people strive for mediocrity. clearly this isn't the case in the world.

5. By decreasing the incomes of working people and acting as a de facto charity, a welfare system encourages people to give less to private charities.

if you are giving money to people through the government the role of private charities becomes less important

How valid do you think these criticisms are?

most of these concerns are purely academic. Although i am a true capitalist, certain aspects of welfare are necessary to preserve harmony in a state. For example, if you don't help poor people, those people will be more at risk to become criminals thereby potentially committing crimes on those rich people who want to deny them benefits. THere is a point where welfare is actually more cost effective than no welfare at all. Without welfare systems the burdens on society from crime, health risks, and civil unrest are potentially crippling.

While studying in Oz I was eligable for Austudy, a govt subsidy of $165 per week. Prior to my study I had a 3 month wait for the course to start after returning from O.S. travel and during that time I was on unemployment benefit for $200 per week.
Effectively you get paid more to do nothing than to further your education to try and improve yourself and your situation.

Once you earn over approx 100 per week through part time work the amount of subsidy begins to reduce more in favour of the student than unemployed, tho I'm not sure to what extent.

Working one 8hr shift a week and studying left me with 300 per week, working 2 shifts only 350 per week. There really was no incentive to work more than 8 hrs, even though I was only doing minimal study hours.