It depends on your frame of reference. If the message succeeds then there will be a window (all else equal and no funny business in the networking, around 1/2 r

It depends on your frame of reference. If the message succeeds then there will be a window (all else equal and no funny business in the networking, around 1/2 round trip time) where the UI displays the message as spinning when it has in fact been successfully sent. If your frame of reference is not whether the message has been successfully sent but whether you know for certain that it has been successfully sent then I think you'd be right to call it "realistic rendering", but from the message's frame of reference "pessimistic" isn't a bad choice of words.