"A lot of people would dispute the premise that ownership is the same thing as power. A recent obvious example of the influence one company can have without owning lots of others is Apple. When Steve Jobs died, people from all over the world – including world leaders, politicians, celebs and the media, as well as the fans – reacted strongly, simply on the basis of the products his company made."
People reacted strongly. Fine. What does that actually mean, on any practical level? It means they liked the toys Apple made. Nothing more. Apple had/has little actual _power_ over anything very important: maybe some small degree of power over certain aspects of how people use their recreational time, and that's about it.
Also, if financial institutions hold the power, they're hardly likely to interfere with something that's making them money hand over fist like Apple, are they? Why mess with a good thing? So Apple doesn't seem like a particularly useful example from any angle. It might be more interesting to look at what happens when a tech company stops making money, or more precisely, when its share price drops. Viz, interestingly, Microsoft - which continues to make money hand over fist, yet due to a _perception_ in financial circles that it's old-fashioned, not cool any more, etc, etc, subject to substantial pressure from certain of its shareholders to change course.
(I'm hardly a Microsoft fan, note. But it is a more interesting example than Apple.)