Being both a railway history nerd and a heritage advocate, I am loathe to speak against something which is an established landmark in the city and which is a well preserved rare example of definite historical value. However.....
My take - many of the heritage buildings that we conserve date from the era before indoor plumbing. When we conserve them, we do not remove the indoor plumbing in the interest of historical fidelity. No one wants to use an outhouse in January, even if that's what people did in the past times we are commemorating.
The Bush trainshed design is the railway equivalent of the outhouse.... while distinctive and technically interesting, they never contributed to a positive passenger experience. Even in the golden days, they were dark, dingy, sooty, and unpleasant.
I am not in favour of razing the trainshed altogether but it could be given a substantial reworking that interprets its values in a more interesting way and creates a new architectural treatment appropriate for a "great" train station.
- Paul
PS - Union Station was not Toronto's first train station. Its predecessors had more interesting trainshed designs. Maybe that would give some inspiration while respecting the heritage.