Those are all well and good, but those are some of the least interesting and least important assumptions that need to be made because they are assumptions that don't materially impact the way you invest your finances. How you manage your finances leads to those assumptions (bonds, assume 5% return; stocks 9 or 10%), not the other way around. The following are what I think constitute a set of rational assumptions that should be drivers of personal finance:
- I have no job security. My job could be outsized (outsourced or downsized or otherwise eliminated) at any time.
- I will never recieve a penny from social security.
- I will pay 35% taxes on my income, and that number will go up over my lifetime.
- My company's pension plan will fail.
- My children will not recieve one cent from financial aid.
These are harsh assumptions, but once you come to terms with them they are quite liberating. It is antithetical to say that you are "financially independent" if you are dependent on someone else for your paychecks, retirement, or for your children's college. If these assumptions don't come true and I do end up getting something from one of these programs, great - found money. But if not, there will be nobody to blame but myself.
Financial freedom is being free regardless of what happens, so I think it is only rational in personal finance to plan for worst case scenarios.