Sidecar championship has been around for about 45 years, a lot longer than the classic!
One regular classic competitor swears the best event of the year is the Mel Harrison sidecar national as it has 'Proper' sections.
Don't think you can say the sidecar championship spoils the classic, i think over 40's on modern bikes are not suited to the same sections as sidecars but real pre 65 solo's (not fiddle bikes) compliment it OK