Apart from objections to feasibility and cost, it is hard to see why anyone would oppose such proposals, as they would undeniably make these densely-populated parts of the city much more pleasant and livable, as well as making all of the surrounding property much more valuable, at the price of forcing cars to travel under tunnels for a small part of their journey. Similar plans have been successfully implemented in Seattle's Freeway Park, Boston's Big Dig, and Chicago's Millenium Park. In New York City, an unused elevated railway was successfully converted to the High Line park.
My question is whether these proposals are ambitious enough. Instead of building decks over freeways, imagine the possibilities of simply dumping the dirt directly on top of the freeway. Instead of placing a park on top of a freeway, imagine replacing the entire freeway with a park. The Hollywood Freeway represents a hideous scar through the Cahuenga Pass and Hollywood. And it doesn't work all that well as a transportation system. In fact, the Hollywood Freeway has the dubious honor of being ranked as the #1 worst commute in America. From personal experience, I can testify that in rush hour it is usually faster to take surface streets from Hollywood to downtown, as the Hollywood Freeway often resembles a giant parking lot. Another alternative to the freeway is a quick commute downtown via the Red Line subway, which travels the same route often at greater speed.
On the other hand, on those rare occasions when the traffic is moving, you can get from downtown to the valley via the Hollywood Freeway in about 20 minutes, and most people who take that route frequently would find it almost impossible to imagine closing it off. But think of the trade-offs. Instead of an ugly, polluting, congested mass of cars destroying scenic passes and historic neighborhoods, we could have bike paths, green spaces, and lots of other amenities. We could create billions of dollars worth of valuable development property, on some of the most desirable real estate in the city. We could encourage a lot more people to leave their cars at home, or dispense with the need for a car for commuting, and save tons of oil, money and carbon emissions. I speak as one who would often be inconvenienced by the loss of the freeway route, but I think the benefits of a more livable and beautiful city would more than make up for that loss.