A WP deployment on a simple shared hosting plan like that could run itself without needed a dev or sysadmin.
Maybe in some cases but that hasn't been my experience at all (or the experience of all the devs I know IRL).
Just a couple of weeks ago one of my clients installed a plugin which didn't allow users to log in.
And then come the legal fees for making the site actually conforming with the law, such as GDPR. Those fees are increased, because of people wanting to do stuff they need to declare to visitors of the site, for which they want reassurance, that all is well.
And then come the costs for paying a dev anyway, to fix things that they break or that become broken over time.
So no, 9.99$/month are very very far from a realistic price these businesses pay.
I'm not saying WP is great. Taking over a WP project from someone else can be daunting in tech debt and weird choices. But in terms of having a simple brochure website for businesses that get < 10k weekly visitors, it's pretty quick, cheap, and easy.
No real maintenance? So either you let your PHP version and plugins become outdated, or you sooner or later have to fix things breaking. Maybe you simply did not notice any breakage, because you don't do maintenance for customers?
A brochure website? Does that mean people enter their e-mail to be sent a brochure? (Then paragraph 1 applies again) Or brochure meaning, that you merely display information on pages and that's it?
I think for small info sites what you describe can be true, but for anything slightly larger not, especially not for small businesses.