The bean counters say the millennials will inherit many trillions of dollars. Basically, there is tons and tons of unspent excess money on deposit. If you have a big bank account, you can’t just bring all that money home, and stuff it in your dresser draws.
You have to store it. So basically, people in the future will have to PAY money or contribute money to have some bank “store” your excess funds until you need them or die and pass it on. Obviously there is bigger supply of money deposited than the demand to borrow it.