1) Here are is an easy to read chart re: stock market returns since 1950.
http://www.dailymarkets.com/stock/2009/08/05/historical-returns-of-the-sp-500/The Bush years were very bad overall for the market, but it's not correct to imply that returns were substantially greater in the 1950s than most of the time since then.
2) Actuaries are well paid to review recent rates of return, funding obligations, ages of covered employees, etc., and determine the levels of over- or under-funding and contributions needed. They do not simply use the 8% that pundits talk about on TV.
3) The whole point of defined benefit plans is that the risk of investment losses is borne by the employer and that the employer is obliged to increase contributions if funding is substantially below targets. If the market does well, the employer saves money by making a lower contribution in that year. Employees don't get a "bonus" as they would in a defined contribution plan.
Employees trade off salary for these plans, particularly if collectively bargained or under individual contracts. (This is one reason why the total pay package for the WI public sector employees is lower than that for comparable private sector employees). Reneging on these agreements after the fact is not permitted in the private sector, governed by ERISA and other laws. If employers want to curtail or terminate the programs, they have to get approval from the IRS and DOL. Ethically, you can't fairly reap the benefits of DB plans but then when times favor DC plans, refuse to pay the piper. And in the private sector, there are legal restrictions on your doing this as well. ERISA (etc.) is stricter on private sector plans than on public sector plans.
4) In many cases, states have gotten themselves into trouble by not making the mandated contributions even when flush. Whose obligation should this be?
5) The primary reason that private sector employers have dropped DB plans is not that there is anything inherently wrong with their design. They existed for many years in many organizations before the Congress began permitting the cheaper 401(k) plans that put less of the burden on employers to bear the risks of poor investment years. Because many employees don't really understand how deferred comp. plans work, or may have changed jobs too frequently to benefit much from DB plans, employers figured they would get more bang for their benefit buck by offering a 401(k) which the employees might have mistakenly thought was "just as good."
6) The aging of the baby boomer cohort is completely irrelevant in the case of defined benefit plans that have been properly funded. You don't seem to understand what actuaries do or how DB plans work. They are paid to anticipate the outflow of benefits given the ages, salary levels, etc., of the workforce, as well as inflows based on these data, and to advise what needs to be added to the fund to keep it fully funded. DB plans aren't "pay as you go." The problem occurs when the needed contributions have not been made over the years including upward adjustments when investments do poorly.
7) Health care costs are certainly an issue for all US employers. But this point and all of the above and your post sidesteps the major issue. In WI and in other states, employees have already agreed to accept reduced benefits and/or that they will also have to make greater contributions, and in many states the pension plans have bounced back from the pits of 2009 (though clearly they are far from regaining the returns they should have enjoyed in the 2000s). But the WI dispute and the dispute in other states is about union busting and demonizing public employees. When governors are giving tax breaks to businesses and making other decisions that further imbalance their budgets, they have no credibility.
I was in this industry for some time. If you don't believe me, do more research.