> When one guy has accumulated large sums of money by promising to invest it only on the basis of yield and then turns around and starts throwing that weight around, he's effectively overriding the collective judgement of many other people who can't cause as immediate an impact on any specific firm as he can.

By investing with an organization, individuals and institutions are authorizing that organization to make investment decisions on their behalf within the stated objectives and requirements of a given fund. In this case, Fink's thesis is that social responsibility will lead companies and consequently funds investing in those companies to outperform in the long run, which is in line with the firm's obligations as a fiduciary. If investors feel like this perspective isn't representative of their individual judgement, they can take their money elsewhere. If investors continue to invest with BlackRock, they're deferring to Fink's judgement on this issue and nothing is being "overridden".

It may not be ideal, but individuals who amass power and then override other's collective judgement is a large part of how our society behaves.

Absolutely so, but that doesn't mean we should encourage it!

Depends on the specific situation. Sometimes it is a damn good idea.