The Province doesn't own this property (it belongs to the City of Toronto) which is in a protected conservation green zone in the ravine. The Moriyama building itself was designated heritage protected in 2023 so it cannot be demolished. Let's not get ahead of ourselves.
From Infrastructure Ontario itself on the limitations of expanding the Ontario Science Centre:
View attachment 717703
via: https://www.infrastructureontario.c...b5/osc-modernization-business-case---2016.pdf
The Province is the tenant and they are required in their lease to give the City of Toronto (the landlord) the option of maintaining the building on site if either party wishes to terminate the lease.
That's not to say that Doug Ford didn't get what he wanted out of this. He pitched an Ontario Science Centre to Ontario Place subway line and then sold the rights to build at Ontario Place to Therme and Live Nation and the rights to develop around the new subway line to private developers. They don't need the Ontario Science Centre lands or building to develop at the immediate intersection surrounding Don Valley Station.
The City of Toronto will inherit the entire Science Centre complex as soon as the lease with the Province is over (2064) but the city can now require an early termination since the Province no longer operates the Science Centre, violating the terms of the lease. The Moriyama building would make an excellent Community Centre for the rapidly expanding neighbourhood.