Lack of differentiation within a tiny elite circle of candidates does not imply that salaries do not reflect individual differential value broadly. While these people control a large amount of capital, they do not
own that capital -- their control is granted due to their talent and can be instantly revoked at any moment. They have no leverage to maintain control of this capital except through their own reputation and credibility. There is no "tenure" for executives -- the "status" of the role must essentially be re-earned constantly over time to maintain it, and those who don't do so are quickly forced out.
The researchers being hired here are just as accountable as the execs we're talking about -- there is a clear outcome that Zuck expects, and if they don't deliver, they will be held accountable. I really, genuinely don't see what's so complicated about this.
Accountability to a business line does not imply that if that business does poorly then every exec accountable to it was doing poorly personally. I'm actually a personal counter-example and I know a number of others too. In fact, I've even seen execs in failing BUs get promoted after the BU was folded into another one. Competent exec talent is hard to find (learning to operate successfully at the exec level of a Fortune 50 company is a very rarefied skill and can't be taught), and companies don't want to lose someone good just because that person was attached to a bad business line for a few months or years.
Something important to understand about the actual exec world is that executives move around within companies constantly -- the idea that an executive is tied to a single business and if something goes wrong there they must have sucked is just not true and it's not how large companies operate generally. When that happens, the company will figure out the correct action for the business line (divest, put into harvest mode, merge into another, etc., etc.), then figure out what to do with the executives. It's an opportunity to get rid of the bad ones and reposition the top ones for higher-impact work. Sometimes you do have to get rid of good people, though, which is true of all layoffs -- but even with execs there's a desire to avoid it (just like you'd ideally want to retain the top engineers of a product line being shuttered).