This is a great question. Let us assume you found a tomb that was 1000 years older than the one at Beni Hasan (where the current oldest trace can be found) and that it has a picture of a person balancing a stick. Do you think that when you took your finding out and exhibited it to the world that it would change our history books and it would become accepted that juggling is now 5000 years old? I do not think it would change anything. I think our understanding comes from a relation to throwing and catching specifically. Not because of history, but because that is what the culture of juggling looks like today. Remember, I am simply asking if you feel the same, it is a proposal.
I am proposing that we see juggling as a relationship to the default form (3b cascade), because I do not think that activities will be seen as juggling if one does not see (or feel) a connection to the default form and the proposed activity of juggling. I think that the connection is an intuitive one, and not a conscious one, up until this point.
This is an unfortunate misunderstanding, and i take partial responsibility for it. I know that I have said something along the lines of "Historically, juggling in it's default form is throwing and catching with more objects than hands". Please let me explain this a bit further. What I meant when I said that, is that more objects than hands is the oldest trace of juggling (the Egyptian fresco at Beni Hasan). However, juggling does NOT come from that, and the word juggling has nothing to do with it.
The origin of juggling is unspecified entertainment. It has then slowly gone from being unspecified, to slowly becoming more and more specified. Dance, storytelling, magic, singing, etc are all activities that at one point were part of the repertoire of juggling. At no point was throwing and catching there on its own.
I do think that there is value in you pointing out the inherent difference between these two categories of activity, and throwing/catching, but what I am trying to describe is that juggling is dependent on relation, and not hard lines, such as continuous motion, skill, manipulation etc. If I follow your line here, I would have to categorize rhythmic gymnastics and skateboarding as juggling, and no one uses the word juggling to represent those activities. They have a clear cultural identity of their own, maybe an even stronger one than juggling (if you would agree that more people know are familiar with skateboarding than juggling).
In the past, everyone tried to define juggling based on hard lines. It always results in a misplacement of activities, to belong in the category of juggling.