Depending on how much money you have you can always make your own index fund, and just reshuffle somewhat once a year. You will have some cost when it comes to broker fees of course with this method.

Diversification is an interesting subject in itself, much happening now in the theory of diversification. Many seem to agree that it is still somewhat valuable but less valuable than before to diversify internationally, with the global village pretty much being a fact of life at this point stock markets tend to over medium long term to move somewhat in unison. This last leg of the US bull market is an interesting outliner, is it beginning of new trend? Will diversify international be worth it again?

Then we have the issue of diversifying in sectors, if you only pick one company in a sector you can end up picking a loser, but if you pick 4 you might own the whole sector, while 4 in another sector is just a fraction of the players.