Disagree. This is the tired old argument of the active investing.
I posted the SPIVA report upthread.
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Investing is a long game. When portfolio managers who are very much interested in investing, supposedly better learnt than average goe picking stocks, on a risk adjusted basis failed to beat the index by 85% to 95% of the time. Also you have close to zero probability of identifying those managers before.
OK from above observed data, 33% seems reasonable to beat the index in one year. But one has to do each and every year. From your example same 15 stocks does not outperform index every year. So cumulative probability of beating index goes down each further year. That is before cost drag and tax drag.
This is terrible advise. One of wonders of investing is the compounding effect and reduction of risk thru' time. If you loose time earlier, one needs to be riskier during their later year which exposes them to "sequence of returns".
I am not against trading, being trader myself. Trading and investing is two different things. For security investing there is no better way to do then indexing.
There is no point in trading same 15 equities as you mentioned. Big cap stocks are researched and arbed to death for any inefficiencies. Trading in my opinion is more about adding another source of return not realized by the "equity risk premium" for stocks or "term premium" for the bonds. Trend trading (momentum), Carry, Short index vol, private equity (liquidity premium) and to some other diversifiers like CAT bonds & micro lending diversifies index investing and make much more sense.