> Don't see how it can "implode" -- the pensions would just get smaller.

Two problems with this:

First, it will get small enough that you won't have a retirement and will work till you die.

Second, reducing pensions tends to be quite unpopular, and when chasing profits for the current quarter, CFO's don't care about the unsustainability of pensions decades down the road. By the time they have to deal with it, it's too late.

Oregon still has a defined benefits pension for its workers, and the problem is here and now. Small towns cannot afford to hire more firefighters, police, etc as their population grows because more and more of the money they have is going into paying out the defined pensions. They keep getting around this problem by taking out more loans to sustain those pensions. Despite the problem being now, they cannot politically reduce benefits for new hires.

> Don't see how it can "implode" -- the pensions would just get smaller.

You are describing an implosion - something getting smaller.

That's "decrease". Implosion is a rapid catastrophic crush, like that Titan submersible.