Middle class folks can't retire "just" on what they have ... they need to beat inflation at least "a little" even after the retire ... 

Pardon the "big" numbers ... if you have $10,000,000 "in the bank" and you can live comfortably on $100,000 per year of cash flow (1% of the $10 mil) ... then you have estate concerns ... but not retirement concerns.

For the rest of us ... pardon the geeky number-crunching .. but we need to earn some kind of positive ROR (Rate of Return) after taxes ... after expenses ... and after inflation ... or we will "run out of money."  (Assuming you have no Guaranteed Joint Lifetime Income ... no Social Security  ... no pension ... no life annuity ... etc.

Speaking of very smart folks, Warren Buffet spoke recently in perhaps more emphatic terms than ever about the SUPERIORITY of Index Investing for the average investor.  He gave what he called a "sermon" about Index Investing during the annual meeting of Berkshire Hathaway ... which was Live Streamed for the first time ever.  ... You can watch the "sermon" on Yahoo Finance .. it begins about 2:42:20 ... Source:  Warren Buffet ...   https://finance.yahoo.com/brklivestream/

“It seems so elementary, but I will guarantee you that no endowment fund, no public pension fund, no extremely rich person (wants to believe that index funds are best).

 “They just can’t believe that because they have billions of dollars to invest that they can’t go out and hire somebody who will do better than average. I hear from them all the time.”

Source:  Warren Buffet   ...  http://www.evidenceinvestor.co.uk/the-gospel-according-to-buffett/

Buffet took particular aim at investment consultants working with institutional investors such as pension funds etc., but by extension he was aiming at a bunch of the entire financial advice industry ... which is another post.

Watch the sermon ... it is worth it!