I'm not saying that, in this scenario, putting all of your money into a 401k fund for 20 years is a good idea either. In fact I think it should be fairly obvio

I'm not saying that, in this scenario, putting all of your money into a 401k fund for 20 years is a good idea either. In fact I think it should be fairly obvious that asking the question implies that "standard investment advice" must be revalidated for its applicability.