So I think this argument actually works against you.
First, nothing can bind future generations. They can always change whatever they want to. In the United States, a future congress can revoke, alter or amend any law passed by a previous congress. So if you want your pensions to be secured, the benefits cannot be guaranteed by faith in future governments!
In this sense, a defined contribution plan is more secure than a defined benefit plan. This is because a defined contribution plan belongs to the recipient as soon as the funds are allocated - even if a future government changes the rules, the money that's already in your 401K is yours!
Consider the alternative - are you saying that future generations are always bound to the decisions of past generations, even though they may not even have been alive or of voting age? If a law is fair and benefits society, that future generation will respect it because it benefits society. If that law is unfair to them they can and absolutely should change that law! How could it be otherwise? As a socialist this should utterly appall you. Do you believe that laws put into place by capitalists and large corporations constitute binding pacts between governments and their citizens, and that any future government that seeks to amend them towards a more equitable balance is now acting unfairly?
If the median income in a nation is $40K, but the median pension given by the state exceeds that income, then who is suffering unfairly if the pension is reduced to be more in line with what the average worker earns? How as a socialist could you justify a pensioner earning more than a productive active worker?