“but the pensioneer has contributed to the pension and is thus protected by ERISA laws.”
If ERISA is a law passed by a legislature, it can be modified or cancelled by a legislature.
Exactly and it not only can be, it must be. There is a limit to the ability to tax full time workers grossing $5K or so per month to subsidize pensions of non workers and retirees making many times more, both politically and financially.