I think you're looking at this from the wrong angle.
> I'd like to see this general rule somewhere. People say it often, and cite a few examples, but nothing that amounts to a general rule that I have seen.
Pick any megacorporation that has reached a point in terms of market cap where they can dictate favourable political/economic conditions for themselves, so from when companies started appearing, so mid-late 1700s. With the caveat that almost all of them have "good" effects from specific perspectives: all huge eastern trading companies (Dutch & British East India), all huge western trading companies (well, sure, they traded slaves but yowza, look at how much they put into the economy!), all huge chemical companies, all large arms manufacturers, all huge oil companies, Japan when split between zaibatsus, etc etc etc. Doesn't seem to matter what sector they are in apart from it involving control of a necessary resource.
> Also, it’s simply not true that these companies only real aim is to generate money from investors.
Yes it is, that's what the point of a company is. It's literally the definition. It's why a company as a legal/business entity is formed. Hiring subject matter experts -- well, yes, obviously a company will do that, and those experts will have personal reasons for working for the entity that are most often not directly be "enrich this entity". But that is for the most part irrelevant to my point.
> And really - the companies we’re talking about have only had this ‘power’ for just over a decade, and frankly all of their products, Google, Apple, and Facebook are quite lacking
Their products being lacking from your perspective isn't really relevant here either. Google and AWS control and sell access to physical infrastructure, virtual infrastructure, cross spectrum products built on that infrastructure, tools to use that infrastructure. They control [a large % of] both ends of the pipe, so to speak. Apple, I dunno, I don't know much about how they got to the position they're in. FB, well as a minor example: "ah well, yes, it was kinda sorta mainly organised on our platform using our infrastructure, but whatcha gonna do?".
And only having this level of power for a decade? Again, why is that important, I'm not sure what that has to do with anything.