The parking can be replaced underneath new towers. Why would we allow it above-grade?
it's thousands of spaces you would be talking about replacing, you get into the financial feasibility of that all, especially since you will have to build new residential spaces as well for the new towers. It's more complex than that.
Not saying it's not possible, but integrating above-grade parking in situations like this makes the numbers work a lot better where possible. There is a reason CF isn't proposing to touch most of their existing garages. The likely best solution would be to "strategically" demolish parts of the existing garages where it makes sense, like right at the corner of Sheppard and Don Mills, to improve the public realm along them. The garages along the 404 for example likely make no sense to demolish as the recovered development land would be marginal and it would be very expensive to replace the lost parking.
There is more complexity here too, like the existing underground bus terminal which likely makes overbuilds structurally challenging ($$$$).
It's a complex site that is likely not going to please the urbanism idealists to redevelop, but that doesn't mean CF can appropriately go plop a useless private drop off and amenity pavilion right on Sheppard and call it a day either.