The Gardiner Expressway had to come down. A decade ago in Toronto, that was the consensus. Urban designers, the new Waterfront Toronto agency and Mayor Mel Lastman all agreed: It was a necessary step to rebuild the city's decrepit port lands, and worth its price tag of more than $1-billion.

Two and a half years ago, John Tory spoke about tearing down the Gardiner as "something that's going to make this a much greater city."

So what's changed? Today, the east end of the Gardiner is collapsing. It needs a multiyear rebuild. And Mayor John Tory is ready to lead city council to spend hundreds of millions of dollars – not to take it down, but to put it back up.

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In post-Rob-Ford Toronto, his position has an obvious political payoff. But if Mr. Tory wants to go that route, he will have to own the consequences. Rebuilding the Gardiner will leave a blight on the waterfront, an area that's becoming the symbolic heart of the city. And it will cost us, in every sense.

Since last month, the mayor has advocated – with arguments that are almost all misleading – for the so-called "hybrid." This is essentially a rebuild of the Gardiner East. He's implied it would facilitate more development than tearing the highway down; this is false. He also couched the traffic argument in grand rhetoric about "great urban design." Building under a highway, he implied, would be the sophisticated, big-city move. New York has done it! Amsterdam!

Yes: You can put things under an expressway. You can also put lipstick on a pig.

Using the space underneath expressways is the kind of underdog move now beloved by young architects; it came up in an ideas competition in Calgary that I covered last month.

On Toronto's waterfront, Underpass Park – which sits under a set of highway off-ramps – got a nod from the mayor this week. Its design is brilliant, under the circumstances. But the nicest part is actually in the sun, between two ramps and not under them. What would make the park even better? Remove the roads.

There is a broad expert consensus that urban elevated expressways are bad news: They're noisy; they are polluting; they are obstacles to a varied, lively streetscape. Ask yourself: Given the choice, do you want to live next to an elevated Gardiner – or not? Would you rather go to a park that's under a highway – or one that isn't?

Mr. Tory is determined to spin shadow into sunshine: to claim the high ground of "city-building" as he does the opposite.

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"It is simply unthinkable that we are talking about, in essence, building a new elevated expressway," says Michael Kirkland, a local architect and urban designer who worked for years with Waterfront Toronto.

Jennifer Keesmaat, the city's chief planner, is clear. At a conference last month, where landscape architects hailed the Toronto waterfront as a global example of good planning, she told me: "The best option to realize our waterfront, and to fulfill our vision of building complete communities, would be to create a beautiful, grand boulevard."

The details have yet to be designed. However, when the landscape architects Field Operations – famous for the High Line, a former elevated railway in Manhattan that is now a hugely popular park – looked at the problem in 2010, their imaginings looked very attractive.

We can, Ms. Keesmaat says, make the new stretch of Lake Shore Boulevard a beautiful place.

And can we improve upon an elevated expressway? "Sure," she said, "but it's mitigation." In other words, you can make the best of a bad situation.

Is that what we, as Torontonians, want? Is that compromise worth paying many millions for? The "hybrid" plan would be much more expensive than the tear-down alternative; the city's figures put the difference at nearly $500-million, and that doesn't account for the loss of value in the planned Keating Channel neighbourhood, which could reach $200-million in the next two decades.

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The "hybrid" places a highway and off-ramps, rather than a boulevard, through the centre of this new district; it scrambles the plan of the neighbourhood and eats up acres of development land. That would involve huge costs to compensate the development consortium 3C for some of its property; the number could be north of $50-million. Meanwhile, city consultants estimate the lost value of the city's own land at $137-million.

What's most troubling is that the proposed benefit to car traffic is small – affecting 3 per cent of commuters into downtown – and may not even exist. There is lots of empirical evidence that eliminating expressways leads people to change their behaviour. Traffic disappears. This is precisely what happened on the eastern end of the Gardiner when it was torn down in 1998. Mr. Kirkland, Ms. Keesmaat and others have raised the examples of cities such as New York, where, as The Globe's Oliver Moore reported this week, the demolition of the West Side Highway caused zero gridlock. I was there in April to review the new Whitney Museum, a polished $422-million building that sits on the edge of the former highway. It is lovely, and expensive, real estate.

The mayor has found one distinguished design professional to advocate his point of view: John van Nostrand, a planner who has been arguing to mitigate the Gardiner for more than a decade, and not winning the argument. He delivered a belligerent performance at City Hall last week ("They didn't do a very good job," he said of Ms. Keesmaat's predecessors), providing political cover for the mayor. And yet, in 2004, he took part in a design study for Waterfront Toronto that favoured a tear-down. Mr. Kirkland, who was also part of the group, said this of Mr. van Nostrand: "He failed to move the conversation."

Today, the entire design establishment of Toronto is on the other side – favouring the tear-down and "boulevard" option. The list includes Waterfront Toronto, Ms. Keesmaat, the city's urban design director Harold Madi, two former chief planners, including Paul Bedford, the current and two former deans of the University of Toronto's Daniels Faculty of Architecture and Design, the urbanist and economist Richard Florida, the Toronto Society of Architects and the Council for Canadian Urbanism.

If Mayor Tory wants to ignore them and chase a political imperative, so be it. But the decision shouldn't be hidden with the promise of city-building. You can put lipstick on a pig, but it's still a pig.