Hmm, I'd never thought about this before, but I think I agree with you. I have had that reaction, in fact with one show (The L-Word) I simply refuse to belive the last season ever existed. I guess it's similar to 'epilogue, what epilogue?' with HP fic. I certainly agree with your first point, we don't have some sort of duty to support the show-makers with our thoughts.
However I don't know if I really would cancel a show. If I did have this hypothetical power, I mean. If I had a show that was on an absolute high and couldn't possibly get any better, would I pull the plug? Or would I let it keep going, fingers crossed and hoping against hope that they'd manage to sustain their present level of awesome? I don't think I'd have the strength of will to cancel it, I think I'd be too desperate for more. It's like you said, canon is a powerful thing, fic and fanon are great and wonderful, but the draw of more canon is too strong.
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However I don't know if I really would cancel a show. If I did have this hypothetical power, I mean. If I had a show that was on an absolute high and couldn't possibly get any better, would I pull the plug? Or would I let it keep going, fingers crossed and hoping against hope that they'd manage to sustain their present level of awesome? I don't think I'd have the strength of will to cancel it, I think I'd be too desperate for more. It's like you said, canon is a powerful thing, fic and fanon are great and wonderful, but the draw of more canon is too strong.