You've just proved again that your issue is people forming negative opinions before experiencing it, not people forming any opinion before experiencing it. You're complaining that people who think it will be "the best attraction ever" aren't heard from enough - so they're allowed to have their opinion, but anyone who thinks negatively of it we hear from too much? Hmm.
Keep in mind that this is the nature of empirical evidence - there are parts of this attraction that people have experienced. I've been to EPCOT and experienced the impact of the giant unthemed Gravity Building on the park's skyline, which I feel is negative rather than positive. I've experienced the Guardians of the Galaxy movies and don't particularly care for those characters or feel they are a good fit for EPCOT. I've ridden Disney's other "roller coasters in the dark" and do not feel that sort of attraction as Disney has historically built them is what EPCOT needs. I've heard how much this attraction costs and know that it's the single most expensive ride ever built at Walt Disney World, which sours me a little futher because that's a lot of money to spend on one ride that, to me, is already having a negative impact based on the things I just listed. We haven't seen many of the elements that are meant to outweigh these negative impacts, and maybe the scale will be balanced out when we do. Maybe all of my early opinions will be reversed on my first ride and I'll want my ashes scattered in this attraction when I'm gone. But until then I can't just magically decide that this will probably be the best ride ever when I've yet to see anything that would convince me I should feel that way, and seen several things that inspire me to feel the opposite.
This does not mean I've already fully decided that Cosmic Rewind will be awful and that there's no hope of saving it - and any rational person knows that. Obviously. It's so silly that this even needs to be said. Are we not adults?
If the negative opinions of others are weighing down your time of these forums, the ignore button is free to use and often very helpful in curating your experience. But people are allowed to have and share negative opinions of how it's going so far, and often those opinions have validity.