I agree with you wholeheartedly until the last point where I differ a little bit.
It can't be a failure solely because there was a longer version of the ride at some point in the planning. I mean at one point in the planning of NFL, the Mine Train was a meet and greet, so the coaster's existence in and of itself is a success over what was planned.
That being said, I think what lit a fuse with some people was the notion that the ride was downgraded. Downgraded at a time when other competitions are upgrading. In addition, with the knowledge that it has been downgraded, other Disney media outlets are/have been trying to spin the Mine Train as an "E+ ticket"..whatever that means.
I agree that negative opinions should be held until after the ride has opened, however, overly positive opinions should be held to the same standard. It sets people up to expect something that it could never live up to be. Also while I agree, the Mine Train is not a direct response to Potter, it begs the question... what is the response? I think that is what has driven the criticism (sometimes too much) upon this attraction; an attraction which personally I feel will be a great addition to the Magic Kingdom.