I've been involved in a couple of those cases, where a large company ran into an issue, and chose to solve it by migrating to something else. And while the issues certainly exist (and are being addressed), the technical reasons often turned out to be a rather tiny part of the story. And in the end it was really about internal politics and incentives.

In several such cases, the company was repeatedly warned about how they implemented some functionalities, and that it will cause severe issues with bloat/vacuuming, etc. Along with suggestions how to modify the application to not hit those issues. Their 10x engineers chose to completely ignore that advice, because in their minds they constructed an "ideal database" and concluded that anything that behaves differently is "wrong" and it's not their application that should change. Add a dose of politics where a new CTO wants to rebuild everything from scratch, engineers with NIH syndrome, etc. It's about incentives - if you migrate to a new system, you can write flashy blog posts how the new system is great and saved everything.

You can always argue the original system would be worse, because everyone saw it had issues - you just leave out the details about choosing not to address the issues. The engineering team is unlikely to argue against that, because that'd be against their interests too.

I'm absolutely not claiming the problems do not exist. They certainly do. Nor am I claiming Postgres is the ideal database for every possible workload. It certainly is not. But the worst examples that I've seen were due to deliberate choices, driven by politics. But that's not described anywhere. In public everyone pretends it's just about the tech.