(I haven't got any personal thoughts but) you're in eminent company as Stukeley thought the same as you.
You can read the ideas of recent excavators from Southampton here
http://www.arch.soton.ac.uk/Research/Avebury/Longstones00/Longstones00.html
they say "our opinion would favour a termination of the avenue in Longstones Field, at or just beyond the cove." but that the evidence isn't conclusive.