Corporations -- indeed most organizations -- are founded, grow, mature, become sclerotic and bureaucratic, and ultimately die.
The time from birth to death used to span many, many decades. Now, many companies go through the lifecycle in a very short time. This has a profound impact on the viability of defined benefit pensions.
What do Atari, Netscape, Palm, AOL, Apple Computer, Sony, Gateway, Compaq, RealNetworks, DEC, Silicon Graphics, Cray and and countless others have in common?
They went from birth to prominence quickly, and then they also declined quickly -- either to death or to irrelevance (Apple Computer, teetering on bankruptcy, was saved in a last minute hail mary when Steve Jobs returned and secured funding from Microsoft)
The idea of most corporations existing for centuries isn't particularly realistic. The idea of working for the same corporation for an entire career isn't particularly realistic either. The idea of the private sector pension as currently defined and implemented isn't particularly sustainable in the long run either.
Written a decade ago, Roger Lowenstein was prescient in describing the genesis of the problems with defined benefit pensions in his classic
While America Aged: How Pension Debts Ruined General Motors, Stopped the NYC Subways, Bankrupted San Diego, and Loom as the Next Financial Crisis
There was a time generations ago when, if "Bob" changed employers every 3-5 years, people would whisper "What's wrong with Bob? Why can't he keep a job?"
Nowadays, if "Bob" DOESN'T change employers every 3-5 years, people whisper "What's wrong with Bob? Why can't he find a new job?"
In the modern economy, young employees need the freedom to pursue multiple careers and job types during their working lives. They need the economic freedom to switch. Pensions are not particularly conducive to that.