1) buying 51% of most companies is not possible without approval from the existing holders of power. The problem is that buying 51% of the shares necessitates buying at least a few percent from the current power holders.
These power holders usually vote themselves some amount of stock so that they can sell it for a comfortable lifestyle without dilution or with dilution limited to the point they don't lose control.
2) Even the 1) ignores the different classes of stock. Stock with voting power and stock without is part and parcel of the landscape these days and guess whose power it protects ?
3) Fiduciary obligation ... really ? How does that work ? (aside from not committing fraud and the like, which is really protected by other laws already) Can you give examples of it in action ? Usually management protects the stock price, for their own self-interest. They do not make sure shareholders actually make money (which is how I'd interpret said obligation).
4) Even where the interests of stockholders and management/board are opposed, I can name dozens of examples of where shareholders sues company cases were decided in favor of management. When not involving fraud, I can name none where shareholders prevailed.
Actually, the shareholders of a public corporation are its owners. If a wealthy investor like Carl Icahn bought up 51% of Apple's shares, he could make it do pretty much anything he wanted: appoint his friends to the board, fire the CEO, etc. Big institutional investors like state retirement funds have lots of influence over companies, sometimes even seats on the board. And if I bought just one share of Apple's stock, I could attend the annual shareholders' meeting and vote on the issues presented there (or I could vote by proxy).
Also, the management of a company has a fiduciary obligation to its shareholders (owners). The shareholders can sue the management for breach of this obligation, and if they can prove that holding billions of dollars in cash is not in the best interest of the shareholders, they can force the company to distribute it as dividends.