While that is true, the overall return depends on the nature of the stocks. If all are growth oriented, bought at reasonable price, GARP stocks, the return could be better than index, despite some fall. But if the same stocks are bought at high prices, the return could be lower than index too.
Also, as market favors different sectors at different periods of time, money moving from one sector to another, even GARP stocks could face low price appreciation, as there is no demand. So from this point of view, allocating a small portion of PF to stocks that may not be pure growth stocks but are available at relatively low valuation, high dividend yield etc may give good return, if they catch the market fancy, but the caveat is, they may never catch market fancy, and may remain at our purchasing price for longer periods.
So if we create a PF concerning all of these points, the overall return could be that of the index.
Of course, as one matures as an investor, the performance of the PF will increase, as one understands the nitty gritty of the market. Also of course, as one matures, I think not everyone can afford to be adventurous, and may very well settle at 15% CAGR, with minimal maintenance, also because the PF may have grown substantially.
All of this is a journey for sure, if we are in for the long term.
Just my thoughts, not yet experienced all of what I have said above.