The problem is that most of the people that remembers and wants an A7/10 won't be alive in another 20 years. Bikes of the 40's and 50's are dropping in value while 60's and 70's are stagnant and 80's and 90's are increasing. Yes you will always have the rarity's holding their value (Brough Superior SS100's and alike) but our common old A7/10's won't.
My insurance policy's on the bikes are market value. Both the BSA's are dropping while my CB1100F is increasing.
Cheers