DB plans were created when generally employee's started around 20 years of age, worked for 45 years and then conveniently passed away about 5 years later.  The contributions they made thru their deductions or wage adjustments probably paid for the entire 5 years of pension income while the company was able to invest the funds and keep most of the returns for themselves.

Now people are starting later in life, work less number of years and in many cases live until their mid eighties or better.  Numbers don't work out now and the investment returns have been less predictable.

The only way a DB pension will work currently, would be if it is backstopped by an entity that can create or tax money to pay for it and/ or run deficits forever.  ie: governments and their own employees.