Whether it's worth it isn't measured by whether it's a huge improvement, it's obvious to anyone with eyes that the current state is better. It's whether it could have been even better had the $22 billion been spent differently.
So no, comparing skyscraper cost to tunnel cost is absolutely meaningful, because if you want to increase green space one way is to spend an exorbitant amount of money digging a highway into the ground, another is to demolish 5 blocks of 10 story houses, build one 50 story skyscraper, and get 4 blocks of public park as a result.
Urban blight around elevated highways isn't some law of nature, it's just a zoning problem, and one that's a lot cheaper to solve than digging the highway into the ground.
So no, comparing skyscraper cost to tunnel cost is absolutely meaningful, because if you want to increase green space one way is to spend an exorbitant amount of money digging a highway into the ground, another is to demolish 5 blocks of 10 story houses, build one 50 story skyscraper, and get 4 blocks of public park as a result.
Urban blight around elevated highways isn't some law of nature, it's just a zoning problem, and one that's a lot cheaper to solve than digging the highway into the ground.