I'm aware of this example, this was discussed earlier on HN [1]. I think it would be reasonable to (try to) enforce O_APPEND in such scenarios, in which case libc might internally open a pipe (configured to passively or actively be flushed to the underlying file, non-concurrently). A pipe would also be more reliable (and secure, though programs in shell pipelines are usually trusted), since it's no longer possible for isolated programs to change the global cursor, which could overwrite other processes' written data.

[1] https://news.ycombinator.com/item?id=38009458

Also, I feel like you are misquoting me, by not including the whole sentence (judging from the subcomments). I implied shared fds with shared cursors are probably a rare use cases, not shared fds in general (as you explained later on). Shared fds, and especially the ability to send fds, are obviously very fundamental for Unix-like systems, and for moving towards capability-oriented security.