"Jealousy" needs to be unpacked.

I agree. My definition is as follows. Jealousy, unlike envy, where you want what someone else has, and possibly resent them because you do not, is the want to restrict the other person's thoughts, actions or choices to prevent some real or perceived harm to yourself.

Jealousy as a smoke alarm

I agree with that. Though my guess that there are more accurate fire alarms out there.

OTOH, I suspect that even in a polyamorous, compersive context, a total absence of jealousy would cause relationships to drift apart a lot more than they do today

Quite possibly, if there is no other feedback available. But first, I am not sure that it is a bad thing. People change and polyamory goes some ways toward making the relationships change with them, rather than becoming a burden. Also, I suspect that compersion does not equal "no-feedback", though. Certainly healthy parent-child relationships tend to be compersive, with plenty of feedback along the way.