Well that was one big wall of text
Jokes aside, though, I agree with most of what you said here.
I believe beta testers are a thing, around here on Tapas. However, I'm not sure how they work and I don't know if they were asked to review this update before it went live. I doubt it, else we wouldn't have to deal with so many bugs.
You're right it's essential to run some tests before the big dive. As others suggested, this sounds quite rushed - which makes me think tight deadlines were a deal-breaker at some point, during the enterprise, for whatever reason.
Agreed. From what I could gather, Tapas has always been quite good at communicating with users, so this sudden lack of information makes little sense. Then again, it might be deadline-induced. I think they realized it when a bunch of people complained about it and launched a feedback campaign in order to deal with the matter. It's not ideal at all, especially since it triggered some great panic along the way, but you gotta give them that, at least.
Deviantart is tossing journals and forms all across the website, but doesn't listen to its users
Easy to brag about being open to some feedback when most of the input they got on Eclipse (and before) has been ignored because prompted by little accounts who had little power in the community. But sure, looks real good on paper.
I admit they do have a button to switch back to the old site, that's true. However, I believe this button is solely here because some features demand it: groups or forums, for example, aren't available in Eclipse mode and they don't want to straight up delete these parts, I guess.
Yep, agreed. Reddit did that too, having a button to switch back to the old site. But then again, it might be because all pages and features aren't yet handled by the new design. Here, everything seems implemented, so why bother with a "go back" button? You see what I mean? I agree this would be better, but they might not feel the need.
In the end, I don't think it's the dev's fault either. We could go with "capitalism bad, prompting people to follow trends... Bad!" but I don't think it's so simple either. It might be a mistake on the head's part, or they were bought, or head changed, or whatever else. They won't let us know why they can't roll back - for good or bad reasons.
Does it matter? I don't think so. We don't need their reasons to judge the website's quality. We don't like it? Let's move out. They improve it? Let's stay. Simple as that. You don't need holy reasons to judge a product's quality, ultimately. ...Which is why my eyes are locked on their future handling of the update. 
Look at me tossing my own wall of text your way, now. >.>