Currently there is a 19 bus bay terminal for TTC planned at Sheppard and 16 TTC + 7 GO/DRT terminal planned at STC. All local service feeding in from McCowan northwards would move to Sheppard. This includes the busy 939 and 985 express routes as well as most Malvern routes. All that's left at STC would be service for Ellesmere (room for 95/995, 38, 133), 134/913 for Centennial College and southwards routes (9, 16, 43B).
As for development opportunities, the city already determine there's not much benefits from the single stop express subway but still pushed forward with it a few years back.
So do they really need to keep STC as "the hub"? Most TTC riders that go through STC today use it has a transfer station, not the mall or commercial buildings. Presto does make it easier for a quick mall visit but the new STC station would be located further away like Yorkdale station connecting through a walkway to the mall. STC would be a regional hub and connection to Centennial/UTSC but the only busy local transit hub when most of the busy routes moved to Sheppard.