In my opinion, Arrow's theorem is more impressive when you have as few assumptions as possible. When the list of incompatible assumptions is large, somebody can say "Well, duh! If you pile on a whole bunch of assumptions you're likely to make the task impossible."
However, one should not be afraid of more assumptions if there exist compelling reasons for each of them. Furthermore, I would not consider 5 assumptions to be beyond the realm of the reasonable.
But, I would be interested where these two different versions came from. If monotonicity was added later...why and by whom?
I am also wondering if one of the assumptions (or a combination of) mentioned at, for example, http://www.math.vanderbilt.edu/~bruff/voting/arrow.html, would imply that the system is also monotonic.
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