What does HackerNews think of JoltPhysics?

There is now also JoltPhysics https://github.com/jrouwe/JoltPhysics which is used in Horizon Forbidden West.

As far as physics engines go: Jolt currently seems to kinda disrupt the decades-long equilibrium, at least as far as I'm aware from my corner of the wood. I wouldn't be surprised if the physics engine landscape looks very different in 5 years compared to 5 years ago:

https://github.com/jrouwe/JoltPhysics

A few years ago, as someone who has worked on physics engines for robotics simulations, I would have agreed with you. But now a lot of people are doing greenfield physics engines for games and having it work out pretty well. There's a ton of established academic and conference literature in the area now and it's not nearly as scary as it used to be.

For example, Horizon: Forbidden West uses a custom physics engine that started out as one of the core dev's fun side projects: https://github.com/jrouwe/JoltPhysics

Physics engines (at least game quality physics engines) are starting to drift in to "solved problem" territory and there's enough literature now that you can get something reasonable going yourself after doing some weekend reading.

Edit to add: Godot has had its own engine available for a long time, so it's not a totally new effort. It's a heavy refactor and a large improvement but the bones for this were laid years ago so some of that technical debt you're describing has already been paid down.