They view the $100-billion gambit for AstraZeneca as a return by Pfizer to the "mega-mergers" of the previous decade that allowed the U.S. company to benefit from huge cost savings and divert shareholder concern over low returns from its research and development. Read More CNBC 25: The last 25 years —What's changed, who mattered... As recently as last year, Pfizer Chief Executive Ian Read had virtually sworn off major dealmaking, wary of the criticism that huge transactions, such as the $90 billion purchase of Warner-Lambert in 2000, had not improved the company's prospects for innovative products.