After making such a commitment to the investment community, a share buyback by PFE would have been viewed as a tacit admission of defeat in executing the Investor Day plan, and analysts would have downgraded the stock, claiming that PFE was unable to find anything promising to invest in.
Such is the tradeoff in making an overarching commitment to Wall Street: it may help to support the share price, but it reduces capital-deployment flexibility to a material degree.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”