You could blame the 10X dev. But it's fair to say that there isn't much talk about how to quantify those kinds of concerns.
It's not that hard to see that your codebase is a mess.
Even if you have no ability to judge the codebase and systems yourself, extreme case, spin that super-productive person up on an independent side project for a few weeks or months and see how the rest of the team grapples with things.
There are a lot of parts of management that aren't hard, but are annoying and potentially unpleasant - if you realize this dev is much less valuable to the long term health of your systems than you thought, that could lead to difficult decisions and a tricky mentoring situation - so they get ignored because managers are either too lazy, "too nice," or just too oblivious.