I'm not sure I agree with your first paragraph. I mean no disrespect to you or your abilities, but being a C-level executive, especially in a large corporation, isn't something that someone can just "train up to". These types of positions really do require a specific personality, specific desires, often a specific ethic (work ethic and otherwise), specific connections, and more. These are the things, along with the fact that due to organizational hierarchy, there are naturally less CEOs in the world than there are entry level workers, that limit the pool.
I'm certainly not saying that all C-levels possess these necessary traits in a positive way, and there are definitely some C-levels that only got where they are because of nepotism or luck, but I also disagree that there is a significant 'stifling' of newcomers. Nearly every company I have worked at has had a specific "track" for its employees to pursue management (including C level) positions, but my experience is that most people just aren't cut out for it (either because they self-selected that they didn't want/enjoy it, or because they didn't have the necessary personality for it). More specific to the tech industry, I've often seen/heard of Silicon Valley companies having separate "Individual Contributor" versus "Management" tracks. Many engineers self-select the IC track because they don't enjoy management aspects.
And that's not necessarily a bad thing, either. Not everyone is destined to be a CEO, nor should that be everyone's goal, and there's definitely nothing wrong with not being a possessor of the negative-in-many-aspects cutthroat ethics that being a CEO often requires. It's not all too dissimilar to how not everyone is destined to be a programmer, and you can't take just anyone off the street, hand them a programming textbook, and turn them into Linus Torvalds, nor should you.