I think there's a major flaw in the argument that "these people could be curing cancer!" To have any hope of doing something as ambitious as curing cancer you presumably have to have an unusually large amount of interest in the subject. Like enough to devote your entire education and career to. By taking jobs at tech companies they are demonstrating that they don't possess this necessary quality. Few people do. And a…
Not to mention talent in the area. People aren't fungible, especially not later on in their careers.
For example, Steve Jobs (pbuh) is not interchangeable with Eric Lander, even though both of them are/were highly creative and driven people who reached amazing success in their field.
The kid who's a natural programmer and working on the latest messaging app or Flappy Bird would never have been happy doing bench work to cure cancer.