Then use John Bogle's rate of return, as shown in this Wall Street Journal article, as an assumption for the pension funds, and start paying up now rather than put it off with interest.
Of course, robbing future generations and destroying the future is exactly what those who control our institutions have been doing for nearly 30 years, and there is no evidence they want to stop. I only hope that in the end, when the consequences are inescapable, people will not be fooled by some demagouge into blaming some powerless group.