On second thought, technically it isn't NTR if the couple hasn't formed a relationship prior to the "theft". If Hinata and Yui don't get a chance to get together at all, there's only a perceived NTR on the part of the fans...
And I agree with your latter sentiment, since as I said, we have an anime base that already showcases chief dynamics. Yuri should remain just the unassailable leader of the SSS, with all the boys clamoring for her as Otonashi and Kanade go lovey-dovey to the side.
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