It depends what you mean by holdings. If you mean a basket of holdings that is adequately diversified, then most people assume it is true because it has (almost) always been true. So while Apple may never again be as high as it is today, a portfolio including Apple and 50 other companies can reasonably be expected to go up in the long term. The simplest explanation for this is that the economy generally grows over the long term due to an increase in the productivity of the workforce due to both population growth and technological improvements.