That the ship sailed long ago is the point. Perhaps I should have stated "circa 2000, there was no other viable site in Vaughan for developing a growth centre." Obviously that's different, if RER is conceived in 1985, but we're talking about planning over the past 15 years.
(And the UGCs were designated in 2006; the Big Move came out in 2008, so I don't know that it's true "no one was thinking about an RER type system" when the UGCs were selected." Whether the ministries co-ordinated properly at that time, I dunno, but RER was under development.)
As it stands today:
-the planned Concord station has relatively little development potential, shoe-horned in between 7 and 407, with a mature neighbourhood to the south;
-the Rutherford has a mature neighbourhood (relatively) on one side and industrial lands on the other. The latter could have been something a growth centre, perhaps, but still an island.
-the Maple GO is in sprawly-sprawlville and adjacent to a heritage district (and the former Keele Valley site/golf course)
None of the 3 compares to 400 acres between 2 400-series highways, centred around a subway and a BRT. 2 of the 3 are nowhere near the Viva system. By the time the Growth Plan was under development (to reiterate my statement) nowhere in Vaughan presented the same opportunity for an urban growth centre. (If Rutherford was closer to Vaughan Mills, I might make an argument for there but it's the only other thing that's close. The area around the Promenade could have been a good intensification centre too, if it had more going for it than a small branch of the BRT).