People are overestimatimating the leverage they have with "cashing out" the pensions, and that's due mostly to the "collective wisdom" over the past decade.
The most important thing to know that is that "pension funding" is a magical number that is calculated, but it has absolutely nothing to do with how much money is in the pension dividied up amongst the participants. Yes, how much money in the pot is a variable in the equation, but not the primary driver of how well funded the pension is or is not. That number is driven mostly by the long term interest rates. Why is this important? Because ERISA funding rules take a snapshot of that rate every year, and that is the assumed growth for the plan.
The only thing "market driven" that affects this calculation is indirect. When they look at the balance of the pension fund, it's a fixed number that they use for the look forward. The plan itself could have returned 5, 10, 15 or even 50% the year before, but all that matters is the money in the pot when the snapshot is taken. The real driver of the funding calcuation is the look forward interest rate. That number gets used, and the actuarial analysis done with the expected draw from the participants. As you would expect, since this is a compunding number, very small changes can cause a wild swing in the "funding level".
Since the crash of 2008, the economy has been on a sort of life support with a very long period of extremely low inerest rates...sometimes approaching zero. When you are cacluating that out 30 years, your funding requirements for the pension can be very, very large. But the actual funds themselves almost always perform better than that.
Typically, when interest rates are low, that means the economy sucks, and the pension funds don't return much because the whole market is trashed. But we've been in this weird negative universe for the past 15 years where the economy has been going gangbusters, the market (and thus the funds) have been doing very well, but the interest rate has remained near zero, thus still driving funding requirements because the "look ahead return" has only been 1%.
That sets us up to where we are today. The funds have actually been doing very well. Interest rates are back to a historical average, and they actually snapped up fairly rapidly. What does that mean to the funds? That means that they went from funded, or even underfunded (requiring companies to contribute money to the fund), to overfunded, and sometimes WAY overfunded.
A mature DB fund, with "average" returns, and "average" interest rates are essentially self-funding. They fund themselves because the returns from the plan are greater than what is required. Aside from the plan administration, there is zero cost to the company. Back when a DC contrubution or a 401k match, it was a relatively paltry amount. These days, in the world of 15-18% DC contributions, that represents REAL money every two weeks that companies have to stroke the check. Going from a well funded DB to DC contributions, in this envrionment, is going to be expensive.
As Sailing said, you can voluntarily terminate a plan, but you have to purchase an annuity for each and every participant of the plan for their accrued benefit. That costs money, and so the threshold to terminate the plan is higher than 100% funded. Usually along the lines of 110-115%.
So, the ironic part: With the increase in interest rates, some plans have become very overfunded. On plans that are frozen, management can't simply skim the money out of the fund. Frozen plan means assets are frozen as well, which means a LOT of money is locked up that they can't get their grubby mitts on. The only way for them to do that is to unfreeze the pension, which means participants start accruing benefits again.