IIRC:
The age 65 was made comon by the German government old age plan instituted at the end of the 1800’s. Yes, 65 was toward the end of when an average worker could still be productive, not"when they usually die". The life expectancy was in the 70’s though, so it was not considered to be a large burden.
Defined benefit plans were great ideas in a stable situation. However - when the number of employees declines, the market goes sour for several years, or the business filling the plan fund goes bad or shrinks - then the plan suddenly becomes a bigger burden. That’s why defined contribution is so much more popular - what you see is what you get, any shortfall from market problems is your problem not the employers. A defined benefit issue too is it is usually based on “best 5 years of last 10” or similar. As a result, a person may get a lot more than contributions should indicate, as pay goes up over a career. Worst, in many plans, especially public sector ones, overtime is included in that calculation. IIRC before NYC went bankrupt in the 70’s, subway workers would conspire to give all the overtime to the soon-to-retire, thus giving them pensions well above their basic wage. Poor planning.
A pension is deferred earnings - you’ve earned it now but collect later. Airlines were notorious for cheating on this - it was easier for the bottom line to promise money later instead of money now… Then when the money was due, postpone yet again.
A fund for defined benefit requires the company to put in enough money to meet its obligations. In Canada, the fund is managed at an arms’ length by a trust company or accounting firm. (IIRC, in USA or Britain it can just be a function of the company’s accounting department). In Canada, the fund must be audited every 5 years, and any actuarial shortfall must be figured in as larger contributions going forward.
Stelco is notorious in Canada as one of the few companies that pleaded money problems as an excuse to defer contributions to their plan - as usual, it did not help and many pensioners ended up with about a 25% cut in their pensions to stabilize the fund after the company went bankrupt.
Not sure what the rules are in the USA, but I understand one trick was for the company to put its own stock into the fund instead of money - with predictable results when the market or the company hit bad times. (Canadian funds are required to do arms length transactions and be safely diversified). Not sure what the US requirements are for auditing and making up underfunding, but quite a few companies shirked their contribution requirements leading up to bankruptcy, then dumped very badly funded pensions of the US body that handles orphan funds. (Airlines, anyone?)
Public pensions (municipal, state, etc.) are the most notorious because the politicians conveniently exempt themselves from the need to have a fund, and pay employees out of general revenue - thus setting up wildly open-ended obligations.
Add benefits into public or private mix and your obligation likely grows even more wildly.
Y2K was the perfect storm that buggered a lot of pension plans - the dot-com boom busted, so the stock market crashed and a lot of funds lost value; meanwhile, the bond market, where funds are usually parked when stocks suck, hit historically low rates so bonds did not perform the way they usually do. Add to that, business and profits were bad, so businesses did not have the funds to top up the missing money…