No, they have not. It's a pyramid scheme. Whatever they put into the system when they worked probably doesn't even cover 5 years of retirement.
If you choose that (or if your legal system determines that), you need to be proactive with keeping the pension system in balance. While you can change what you promise in exchange for future contributions, substantial changes to what you have already promised can easily require constitutional amendments.
Paid contributions should only ensure that you get a part of that, a bit more if you paid more, less if you paid less.
If workforce/economy halves when pensioners double, you will get 4x less than you "expect".