If you look into the actual historical basis of shareholder primacy, you'll find everything is a bit more complicated than you think it is.
Shareholder primacy has an old idea, coming from a the 1919 court case Dodge v. Ford Motor Co. The facts of the case were as follows:
1. The Dodge brothers - minority shareholders in Ford Motors, wanted to set up their own automotive company. They planned to use the money from Ford's dividends to support their company.
2. Henry Ford would have liked to keep his monopoly on affordable cars. He cancelled the dividend and claimed he's going to spend the money on improving society (by selling the Model T even cheaper) as a thinly veiled excuse.
3. The courts in Michigan saw what's happening and forced Ford to issue dividends.
As you can see in that case case, it was the lack of shareholder primacy that enriched the powerful. In the last 20 centuries of human history the powerful have always crushed the little man with the excuse they are serving the public interest. Beware of anything that empowers powerful individuals (like corporate CEOs and directors) to take arbitrary actions in the name of "social responsibility".