The tragedy of Enron was the automatic investment plans in the 401k for most employees. The default choice was Enron stock. Also, depending on the nature of the matching contribution, Enron stock might have benefited from a larger matching employer contribution percentage or discounted share price. This is why 401k plans now offer various choices that are suitable and diversified for most people.
http://blogs.wsj.com/moneybeat/2014/07/04/are-you-stuck-on-y...
A defined benefit plan could have prevented this tragedy. It's funny though. Defined benefit plans are mocked at by large firms but increasingly being used by wealthy individuals/families and their small businesses. Nothing beats government subsidized PBGC insurance for the future retirement plans of America's job creators.