All of those are what I would consider static style or tempo marks, and I don’t dispute that at all. It has always been the convention for that to be above the staff.
Yes, it seems the great majority of big band charts written before modern notation programs forced a different default have those gradual tempo changes below the staff. Those (rit, rall, a tempo etc.) are really the only items I’m commenting about / objecting to. And I don’t really object to “a tempo” being above the staff. If the performer observes the rit/rall correctly, then the “a tempo” is obvious, regardless where it is placed.
My motivation is to have music performed as well as possible on first reading. IMHO, this aim is better met by placing those particular items below the staff which has been a rather solid convention for a long time. But I am also one who will almost always replace gratuitous scattered 16th notes with staccato 8th notes to make them easier to read and therefore more likely to achieve the musical intent on the first reading. I think there is often a struggle between the composer’s concept of elegance versus the performer’s need for pragmatics. It is like re-spelling double-sharps and double-flats. Purists will object, and players will thank you.