lol, the internal docs on this at Amazon were something very close to "we invented this before Avro and we think that's probably a better choice if you need binary serialization."
I have never used Ion so I cannot speak to its use in practice, but I haven't really had too much of an issue with msgpack. It's faster than JSON, more compressed than JSON, without being any more difficult than any JSON library I've used. It's an almost-universal good for me; the only thing you lose is the ability to easily introspect the messages if there's an issue.
Honestly, if you’re in a case where you absolutely know none of these work for you and you can absolutely prove you need another, you’re probably just going to write your own. And that’s a fleetingly rare case.