If AV falls, the idiots will say "That's settled then. The people have voted for FPTP. End of."
And we will be stuck with FPTP for yet more dreary decades.
If AV, for all its pusillanonimity, wins, the FPTP log jam will have been broken.
This will be a symbolic change.
We can then campaign for its upgrading for AV+, a kind of PR, which will mean that the will of the people is represented in Parliament.