You're right in as much as it is only currently the biggest game. But you underestimate how important current is, and are completely misleading the global appeal of the relative fixtures. Last years Manc Derby had the highest television audience for a Club game ever, and only the World Cup Final in 2006 had a bigger one.
Ah Mr Schofield you have to be very careful with numbers. Now go away and do some research into which game achieved the highest REVENUE worldwide.f! As for being current then look back at just how long the Liverpool:United game has held that title. It's not just a one-of