The option was either this rapid bridge replacement (RBR), or a whole new bridge.
The RBR reuses the existing abutments and piers. The new bridge is built off to the side, and then with a weekend closure of the 400, the old bridge is removed and the new one brought into place. (Similar to what was first done in Ottawa:
https://www.canadianconsultingengin...nt-of-highway-417-island-park-drive-overpass/.
A new bridge would have been built in stages and had all new components. The span would have been a bit longer to accommodate additional lanes for the LRT and still maintain car lanes. Prefab components could have been used to speed construction, but it would likely have taken 2 years. Lanes would have been shifted on 400, but basically all kept open.
I think the first option had the lower initial cost and the 2nd had a lower life cycle cost. The City said there was no benefit to having additional lanes under 400. Likely in 50 years when this bridge need replacement we will regret the decision to not build a fully new bridge. For the next 50 years, we will regret the decision as going under the Finch Bridge will become a larger bottleneck than it is now.