Ok, but what if you want to transmit two streams? You'd get something like Interlaced-newline-delimited-JSON. And so the list of required file formats knows no end.
I don't see why this is a problem with ndjson anymore than it is a problem with other stream text encodings (like CSV).
ndjson seems to be about how to format the content of the stream, not about how to multiplex multiple streams over a single channel. Use http2, zeromq, raw tcp or whatever else you like for that.
The only reason people care about newlines for a stream is because they've arbitrarily chosen to fetch bytes from the stream until the next newline sequence (readline instead of read). But you could just as easily look for a different sequence, like the ASCII record separator character which was invented exactly for this task, and then you wouldn't have to destructively strip newlines from your input.
We tell our customers our streaming API uses jsonlines for documentation purposes, but we actually just decode in a loop until EOF, and 400 at the first decode error. No separators necessary at all.