Wikipedia uses a good definition for pyramid scheme:
A pyramid scheme is a business model that recruits members via a promise of payments or services for enrolling others into the scheme, rather than supplying investments or sale of products. As recruiting multiplies, recruiting becomes quickly impossible, and most members are unable to profit; as such, pyramid schemes are unsustainable and often illegal.
I don’t think that describes a 401k. I don’t get money if I convince you to open a 401k. I do pay fees to a company to manage my 401k but I’ve chosen the lowest fees available to me.