Look at how much the the Fortune Global 100 list of companies has changed in just 10 years. This highlights why I think individual stock picking is dangerous for most people.
The top 100 is always changing, there are always companies growing and shrinking and trying to predict this is very difficult. This is exactly why you need to buy 1000s of companies not 10's, and you need to buy small and medium companies not just the 100 biggest. It’s those mid-sized companies that will grow to be in the top 100 list in the next decade. Buying a FTSE 100 or S&P500 tracker is NOT a diversified portfolio.