Classic forums move the thread to the top each time someone responds, but they don't thread the comments.
This seems like a much simpler solution, which is much harder to game. Yet I've never seen a site actually use it in the wild. How come?
In a classic forum, since posting something not only moves the thread to the top, but puts your post in a highly visible place (the end of the thread), it's a lot harder to have too many deeply branching side conversations with low value replies: there's a social norm against polluting threads.
For something like HN where comment trees don't get too deep and the replies are always meaningful, I think threaded bump ordered ranking could work.
Before:
├── a
│ ├── b
│ │ ├── c
│ │ └── d
│ └── e
└── f
├── g
│ ├── h
│ └── *i* <- this is new
└── j
├── f
│ ├── g
│ │ ├── i <- moved to top of its thread and its thread moved to the top
│ │ └── h
│ └── j
└── a
├── b
│ ├── c
│ └── d
└── e