That whole model dates to before automated testing was even really a thing, and no one knew how to do QA; your QA was all the people willing to run your code and report bugs, and that took time. Not to mention, you think the C of today is bad? Have you looked at old C?
And the disadvantage is that backporting is manual, resource intensive, and prone to error - and the projects that are the most heavily invested in that model are also the projects that are investing the least in writing tests and automated test infrastructure - because engineering time is a finite resource.
On top of that, the backport model heavily discourages the kinds of refactorings and architectural cleanups that would address bugs systemically and encourage a whack-a-mole approach - because in the backport model, people want fixes they can backport. And then things just get worse and worse.
We'd all be a lot better off if certain projects took some of the enthusiasm with which they throw outrageous engineering time at backports, and spent at least some of that on automated testing and converting to Rust.