For my part, I don't think this risk is that great. The majority of defined benefit plans are found in large, highly unionized companies. We know that just over 9% of paid workers are covered by a defined benefit pension plan.
If a company wanted to migrate to a defined contribution plan, this would therefore need to be done through a formal negotiation process. If the parties agree to that change based on their right to free bargaining, that's okay and we'll let them make that decision.
However, I don't see why companies would suddenly decide to migrate to a defined contribution plan, because there are risks. If the risks scare them, they just have to fund the pension plan. However, it's not that simple and they won't want to fund that pension plan, invest money in it. They will assume, by analogy, that they can live with that risk just fine.
You can't not invest money without it raising risks later on. At some point, you have to be consistent. If a company doesn't want to take risks, it has to fund its pension plan. However, the legislation allows pension plans not to be funded. So let's accept the current state of the law and the legislation.