While I broadly agree, I do get the impulse, which is not dis similar to Vegas adding sports teams, or more 'family friendly' stuff....
Its the idea of being a well-rounded destination, by which they mean, something for everyone; and also an ability to some degree to even out demand through the year. (though more outdoor attractions in our climate would not be the logical answer on that front).
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On amusement parks, its worth noting that on our side of the border you have Canada's Wonderland, and really, Marineland was the distant second (more like sixth, with the intermediate entries missing, but I digress)...
Where the U.S. side, with a considerably smaller population has Six Flags Darien Lake as its Wonderland counterpart, and then 'Fantasy Island' as much smaller secondary park.
There is almost certainly room for a second, lesser theme park in the GTA. Whether or not it should be sited in Niagara is a different question.