If they can put together an impressive enough attraction, it'll work. Again, Miniatur Wunderland is the most visited tourist attraction in all of Germany. I'm not saying these guys are going to equal that here, but I'm saying that the possibilities are greater than the naysayers here are.
I think Toronto is in need of more family attractions these days, especially year-round, centrally located ones. As stated above, Cullen isn't the best for comparison for a number of reasons, but even then we have to take into account that the GTA has gained over a million people since Cullen closed and our tourism numbers are much higher these days.