I turns out that usually (not universally!) the problem isn't that the previous person did a bad job, but rather that they made a set of tradeoffs that made sense at the time, and make less sense now.
Or it could be that you work with uncharacteristically incompetent engineers, but that's not the first place to look.
And the tradeoffs did not make sense. This was a simple case of bad normalization - a missed many to one relationship. It just so happens that it happened to be a relationship at the center of a very large platform, and is baked through many services. And without fixing it early-ish, we'd be fighting the schema for the next 10 - 20 years and creating unmanageable complexity in the process, all the while making it harder for ourselves to eventually fix it.
So I'm just pointing out something factual. Sometimes making sure something doesn't happen again means calling a spade a spade. If that means pointing out a mistake, then we absolutely should.