If your 401k isn’t earning enough returns to let you retire, neither is your employer’s pension fund, and it will become insolvent. The difference between pension and 401k is unlikely to be make-or-break, and when it is, it’s more likely to break the pensioner (corporate bankruptcy, taxpayer sticker-shock, etc).

Pensions are great when they work, but it's impossible to predict solvency at retirement age. It's one of the most under-reported financial travesties, but your pension can be taken away when you need it. Even the public pensions are targets these days.