The number of women in top jobs has, in fact, fallen sharply.

“I hate seeing her step down this particular year because we’ve had a 25 percent drop in women C.E.O.s at major firms, and this is a big loss,” Jeffrey A. Sonnenfeld, the senior associate dean of leadership programs at the Yale School of Management, said of Ms. Nooyi.

In some cases, the women at the top have been pushed out by activist investors seeking a shake-up in the executive suite. “A lot of women have been easy targets when their performance weakens,” Mr. Sonnenfeld said. “Maybe unconsciously, institutional investors get weak-kneed and don’t back them up and boards of directors collapse like lawn furniture.”

Others tie the lack of women at the top not only to the choices of individual executives, but to a wider culture that is biased against women in the workplace, some experts argue. Women struggle as they adapt their careers to the demands of child-rearing while they are often viewed differently because they do not fit the mold of the leaders, overwhelmingly men, who preceded them.

“We have found that overall, we are stuck. We are seeing very little movement,” said Lareina Yee, senior partner at McKinsey & Company in the global technology practice and co-author of the report Women in the Workplace, which found that only one in five of the most senior executives at American companies is a woman. “We see that entry levels don’t look that bad, but women are not promoted at the same rates as men to the first manager role. We start to see the imbalance of opportunity early on.”

Ms. Nooyi’s impact on PepsiCo cannot be overstated. Over the past decade, she has transformed the company dramatically, expanding its presence in international markets, and also shifting its products increasingly into healthier beverage and snack choices.