Did you ride the Skytrain lately during peak hours lately? There's plenty of "ridership" to justify the service. Even on weekends on the Expo line, it's often standing room only. And the Canada line has exceeded ridership expectations from the beginning.
And your statement that Vancouver is the "only city in the world that plans for light rail metro on routes that do not have the ridership to sustain them" is just plain wrong. New York City built it's subway into relatively vacant areas of Queens, Brooklyn and the Bronx with an eye to future high-density development, which quickly followed. In Calgary, development followed the construction of its light rail system in the 1960's.
The problem with the Skytrain isn't lack of ridership — just the opposite. In the twenty 25-plus years since Expo line was first built, the high-rise development has been built all along the line, to the point that the system can no longer handle the ridership levels. If you don't believe me, try getting on a downtown bound train during rush hour at Broadway station. It's already a sardine can, and is only going to get worse, especially with a lengthened Millenium line feeding into an already overcrowded Expo line.