The challenge with that approach is memory: trigger conditions, if added irresponsibly, can result in unbounded memory and (depending on implementation) potentially linear performance degradation of filesystem operations as well. Unbounded kernel memory growth leads to stability or security risks.

That tradeoff is at the root of why most notify APIs are either approximate (events can be dropped) or rigidly bounded by kernel settings that prevent truly arbitrary numbers of watches. fanotify and some implementations of kqueue are better at efficiently triggering large recursive watches, but that’s still just a mitigation on the underlying memory/performance tradeoffs, not a full solution.