Not sure why you think Murray wasn't competition for Fish at Cincinnati.
However I agree with you that both Montreal and Cincinnati this year were probably Fish's best chances at finally winning a Masters. But he choked away a chance to beat Djokovic in Montreal and seemed to have run out of gas when he played a slightly sub-par Murray in Cincy.
But I doubt he could have become better than Roddick. Unlike the latter, he never looked likely to win a Slam or even make a Slam final. It's true he lacks Roddick's monster serve (most players do) but that said his own serve is not bad and I think his all round game is good enough at its best to have garnered him at least one or two Masters titles and at least a Slam semi or two if not a final. IMO it's his lack of consistency and/or fitness which lets him down (as it does with many otherwise good players). Even with the good results he's posted this year, his consistency and fitness started to become problems again by the time of Cincinnati and have not picked up much since.
Of course age is now against him (he turns 30 this month). I certainly wouldn't put it past him to squeeze out a Masters title before he's done, probably on the North American circuit, most likely IW a la Ljubicic or Miami, ie. the tourneys where he has traditionally posted his best results.
Of course Murray was competition but out of those 2 Masters that is all he had as competition really since Djokovic was in awful form in the Canada final. Murray wasnt even playing great in Cincinnati and Fish has done well against him in the past. Yet he still couldnt win even one, suggesting even a Masters title while in his best form ever, and a wide open situation at his hands, is still beyond him.
I guess he could win one but I would be surprised. I hope he does though, as he has lost 4 finals so it would be well deserved if he could. I agree with your assessment on what he could have been capable of. Yes he could have done a bit more, and spent alot more time in the top 10 or top 15, but he was never going to come close to Roddick's career- a year end #1, three years in the year end top 3, 9 straight years in the year end top 10, 5 slam finals, 5 Masters titles. It is ridiculous to even suggest Fish had the potential to have a career like that.
To those who say Fish has the better overall game (which wouldnt neccessarily mean he could have been better as many guys have a more rounded game than Roddick) that isnt even really true. Roddick not only has a much better 1st serve, but much better 2nd serve, at his best his forehand is light years better than Fish. Heck even today one could say his forehand isnt worse than Fish, Fish never had a great forehand. Roddick at his best moves and defense better than Fish does, even with all the weight Fish has lost. Fish has a stronger backhand and better volleys, and maybe returns better (although as Crazy Man said the Roddick of old returned better probably), and what else. That is pretty much it. He definitely isnt a smarter player, better competitor, or better at the intangibles of the game either.