I'm not sure I understand this line of reasoning. The proposed indoor theme park that makes an attempt to blend with and improve many outdoor amenities is bad, but the abandoned, entirely outdoor, and seasonal theme park that was previously in the same location at Ontario Place was good?
Having year-round space that tries to integrate with its natural setting seems infinitely better to me than space that isn't attended half the year.
I'm not sure why those would be the only two options.
No one has suggested they leave it as is - only that this plan is not an ideal use of the space, especially given what it was intended for.
The plan certainly has its merits, and I generally don't mind having an indoor theme park. This just isn't a great spot for it.