Instants are important if you migrate something, like TLS certificates, data flows, etc. ISO-8601 is widely adopted, unambiguously interpreted, human-friendly format for that. I see no reason to not have it.
The reason for not having is simply a matter of specification size. ISO-8601 is full of weirdness! SOML's date spec is reasonable looking but there is still a lot of nuance.
Having durations is nice and instants are highly complementary. My feedback is just that if lots of literal formats are desired, maybe an extensible literal format!