It seems silly that a study was commissioned for this. Isn't it obvious that a stock portfolio would maintain a higher purchasing power than a bond portfolio.

I think the point of the study was to evaluate the income generated from a dividend paying stock portfolio versus a bond portfolio. As I work with investment clients, they have a hard time accepting the lower dividend income generated from a stock portfolio versus the higher bond yield income. At the beginning of a portfolio construction process, bonds nearly always yield more than stocks. Over the long run the growth of the dividend income ultimately surpasses the bond yield. Then looking at yield on equity cost, ultimately has a higher yield than bonds. The difficulty for clients is to get them to accept lower dividend income today versus the bond yield. Say a ten dollar stock pays $2 and grows the dividend 10% per year. In 10 years the dividend will equal $5 and yield on cost will equal 5% (near a bond yield). Clients have difficulty seeing this; hence one of the reasons for the study.