I'll state my position up front: I have a certain affection for the old Gardiner with its its brash function over form ethos and clean if slightly crumbling lines. But that said, I think that the best solution would be to take down the eastern portion because quite simply, such a big road isn't necessary here; it is expensive to maintain, and its removal would have both a symbolic and highly practical value for Toronto in its effort to develop more sustainable and people-oriented spaces. It might help change some peoples thinking about roads and driving, and as I hope you'll read further on, this is important.
First the history: to understand why the Gardiner is unnecessary east of downtown we have to go back to the transportation planning of 1960s Toronto. At that time, in addition to building a good subway system, Metro Toronto planned to build an extensive network of urban expressways. This was seem as a balanced approach, as few cities in North America were investing in public transit at all. The plan included 4 expressways into the downtown core. Two of the these, the western leg of the Gardiner and the Don Valley Parkway (DVP) had largely been built by the early sixties and the other two, the Scarborough and Spadina Expressways, respectively from the east and Northwest, were moving through the planning stages, awaiting the funding that would become available once the Bloor-Danforth Subway was finished. The Gardiner was built in stages moving further east across the waterfront to accommodate this plan. Because the Gardiner east of downtown would have to eventually carry traffic from both the DVP and Scarborough Expwy., it was built eight lanes wide with a large interchange at the DVP that could be upgraded to accommodate both west and eastbound ramps. Obviously the Scarborough Expwy. was never built. Following the cancellation of the Spadina Expwy in 1971, the province was out of the urban highway funding business, and without provincial support, Metro Toronto could not meet its ambitious highway building goals. The Eastern Gardiner therefore has always been something of a white elephant. There was never enough traffic from the DVP or Lakeshore Blvd. to fill its lanes. Its been something nice to have for downtown-bound drivers from the east but strictly speaking an urban boulevard, as proposed in the replace option could do the job.
So on this level the debate seems simple. Remove an overbuilt elevated urban highway that is both a blight and nearing the end of its design life without major rehabilitation. But, as with many debates centred around the notion of old car-based thinking and new vs. more progressive urban planning and design ideas such as place-making and sustainable transportation, I feel that there is a major piece of the context that is missing. Put simply, our economic and social values really haven't changed all that much since this highway was built 50 years ago. As a society, we prize individual consumption and mobility that favours a primarily private car based transportation system. We may acknowledge that there are spatial and aesthetic problems with cars and highways in dense urban areas, but in terms of collective behaviour, our preference for the speed and comfort of driving remains much today as it was when this section of the Gardiner opened in 1964. In fact, one might argue that we value private consumption more highly today, enhanced as it is by much higher levels of affluence and growing income inequality that inhibits how we value shared public spaces and functions such as transit. This is reflected in public policy; since the tide turned against urban road building 40+ years ago, there has been no corresponding effort to curtail car ownership, driving, or provide alternatives in the form of major investments in urban transit. In a democratic society, this has not been an accident: we may not want a freeway in a our backyard, but we'd still rather drive, even if it means sitting in traffic much of the time (it is still typically faster than our underfunded transit system). It is not a surprise to me that our current mayor, and many others, prize these relics of urban transportation such as the Gardiner, and feel they are necessary. For a society that is still based almost entirely on private car based mobility, and that socially and economically encourages it, they are not relics at all.
I have made my position clear; I favour removing the the eastern Gardiner (also as a resident and condo owner in the Distillery District I have a vested interest in this). From a pure cost-benefit perspective, this solution makes far more sense that maintaining a massively overbuilt roadway indefinitely. I am also a driver, and I admit I love my car. I drive it because it is often the fastest and most comfortable way to get around, even downtown. Transit is often a slow and frustrating experience in this city mainly because it has not been a priority of any level of government for many decades. So I respect those for whom the urban expressway still holds a certain allure, even if it is largely unworkable in practical reality. Changes in thinking and behaviour are slow to come and over years after we stopped building new Gardiner Expressways, we still haven't quite figured out what we want.