You assume that board members can be fired,
Sorry, I wasn't perfectly clear. The ones proposing this at the board meeting would normally be execs, who could be fired.
What if that "one solitary idiot" is the single largest shareholder (directly representing himself)?
Somebody whistle-blows this (one or more of the original scientists, some middle manager, an executive, or a board member) and gets themselves a tidy book deal and is the hero who fought back to cure cancer. But even that's unlikely to be necessary - again, we're imagining human behavior that's very rare, and even more rare for someone who's very successful. Even if someone privately thought this would be profitable to bury (which it almost certainly wouldn't be), they'd have to be non-functionally psychopathic to actually suggest it out loud.
Look, I fully agree corporations (and people in general) do bad things. You can get a weird group-think going where everyone does something that nobody individually would do. Humans can do all sorts of bad things when they're scared. Many times you can get a horrible result through a chain of events that are all, themselves, reasonably innocuous. People will go a long ways to rationalize their own failures. Few people respect the categorical imperative; they'll do things that contribute to problems while denying their own responsibility or moral failure.
There's lots of mechanisms that result in bad behavior - but almost all of them share some characteristics; in very few do the functional people involved feel like they're actually doing something importantly wrong. In the case of a "burying the cure for cancer", that's a very hard leap to make, and suppressing a cure over time is going to take a large number of those leaps by a lot of people (and many of these steps don't make any business sense, let alone moral sense).