The total value of the system is on the order of $18 trillion dollars, slightly more than the ~$15 trillion annual GDP. https://www.ici.org/pressroom/news/ret_10_q4
Too, employees have far more at stake, and far less negotiating power, than employees, a fact quite openly recognised in Adam Smith's Wealth of Nations (Book 1, Chapter 8: https://en.m.wikisource.org/wiki/The_Wealth_of_Nations/Book_...)
There's further a great deal of other infrastructure which reinforces the existing system, from tax and other laws based on employer-provided benefits (including substantial advantages to both employers and employees participating in such plans), and of the services industries built around these (it's far more economical, and actuarially defensible, to deal with groups of people, particularly large aggregates, than one-on-one).
Which means that:
1. You cannot simply turn this ship on a dime. It's quite literally larger than the economy.
2. Changes to present employee benefits represents a material change in compensation and risk profile to those employees, for which they're poorly equipped to either address or organise against.