(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.
All the evidence from the last 6 years of excavations by the negotiating Avebury project shows that the Avenue ended at the longstones cove.
Geofiz and expolatory digs at Fox Covert and Knoll down revealed no stones or sockets.