Personally, no, I don't think Diamondback will always be the tallest at Kings Island. The direction Cedar Fair is taking is one that shows a significant balance between top-notch roller coasters and family friendly fun. On one side, you have Carowinds, who will be building Fury 325, which will be the world's tallest and fastest non-launched roller coaster. One the other hand, you have parks like Cedar Point and Valleyfair who just this year have added several family rides such as Northern Lights and Pipe Scream. Seeing that Kings Island just received Banshee, I think it might be 5-7 years until we see our next big coaster... but when we do I suspect that it will be a giga, and it will hopefully be one of the best, if not the best giga out there.