Theory? It's a known entity that passive funds beat most active funds year after year. Have you ever heard of SPIVA? They release a report each year comparing passive funds to their passive funds vs. their active respective benchmarks.
https://www.ifa.com/articles/despit...active_funds_fail_dent_indexing_lead_-_works/
Most of the categories are mind-boggling....in all Domestic Funds, over the last 20 years, active traders underperformed their benchmark 95.39% of the time. It goes on and on like that through all of the different types of funds. Not always 95%, but most active managers underperform their benchmark index year after year. Warren Buffett, who I think we'd all agree is one of the greatest investors of all time, has said this many, many times. Here's a quote from him from 2014: "Huge institutional investors, viewed as a group, have long underperformed the unsophisticated index-fund investor who simply sits tight for decades. A major reason has been fees: Many institutions pay substantial sums to consultants who, in turn, recommend high-fee managers. And that is a fool's game." (2014). And when he set up the Trust for his wife should he predecease her....he instructed it to be set up like this. "90% in a low-cost S&P 500 index fund and the other 10% in Short Term Government Bonds.
There are millions of investors out there who don't understand how the markets work...paying unnecessary fees that will put a major dent in their nest egg years down the road. When all they needed to do was determine their personal risk tolerance and build a very simple portfolio with very low cost index funds with a company like Vanguard. It's really not that complicated.