It's only partly true, even of the extremities. It would be more accurate to say the stations are being rebuilt. Some could have been reused more cheaply, but they were many years into their lifespan, outdated, and not perceived as very adequate. Most of the running way is simply having tracks laid on it, with little in the way of digging up. There was minimal infrastructure in the downtown where the tunnel is being built, and most of it will remain for buses.
The bus system had little or no growth capacity, and Ottawans rejected the idea of leaving buses for the large majority and building a different LRT line that would serve a small minority. Operating costs will decrease.
Lastly, Ottawa controlled costs in a way that Toronto has never tried to do. The Ottawa project is nearly twice as long as the Scarborough subway extension, costs much less, and will carry far, far more people.
