The vast majority of books frankly suck.
I say this as an aspiring writer and as someone who (as part of that) has critiqued a bunch of books and parts of books. Even published books can be mediocre or bad. And this is even more true now in the days of self-publishing, where there's basically no barrier to pressing "go" before you're ready.
What I see in the central table in the post (a couple paragraphs down from the top) is a power law distribution: at each successive level, roughly 10-30x more titles are able to get there. Sure, some percentage of those successes are due to a pre-existing platform. But how many writers are truly writing at the level of quality of the best-selling authors? I know, for my part, that after doing half a dozen or so major passes on my book, I still get critiqued for a variety of issues, some of which are embarrassingly basic.
Look, I'm not saying writing is a great way to make money if that's your primary goal. But I do think there's more correlation here between quality of writing and sales than most people give credit for. It takes a lot of work to get there, so most people just don't. But that's not to say that the opportunity doesn't exist.
I do appreciate the thoughts on alternative platforms though. Just because the journey is hard doesn't mean I shouldn't be trying to maximize the money I can make along the way. :-)