I understand. So what you want to say is, that the Ocean design is better for accessibility, plus more consistent in general. I ignore the last one, because that does not matter for this discussion (other designs could also reach this by choosing other rules).
To my background, I am more into game design, but some rules are universal. So sorry to not bring up studies, but my experience. When designing accessibility systems, you can make a lot of mistakes, especially when forced to the whole user base. The new design has advantages in terms of accessibility, because the icons have less complexity and the lines are smooth and therefor easy to read. I value that much, donāt get me wrong. Accessibility is important! But it also comes to a cost.
The new design is very generic and boring. If you show me an icon, I could not tell if it would be Adwaita, Ocean or something else that follows these studies. So it can hurt the first impression of the DE. Right now KDE is becoming a lot of new users, especially since all the gaming distros are using Plasma. Unique designs will be remembered more easily. And also the feeling will be different. It is a bit complicated to find the right words, but more accessible designs are looking like the illustrations of a children book with easy shapes etc. Even if the book would contain adult content, most people would probably prefer a more mature look with more details etc. So it seems studies often forget some important parts or just ignoring them, because they focus on a specific topic as accessibility.
In additional about the coherency, GNOME is also using huge rounded window edges (and many other elements) which match the huge rounded icon-lines. On KDE little rounded edges are used while the icons have huge rounded lines. The coherency win is going to GNOME here. I agree with bunnikyuube about making some icons more round if that is going into the direction I previewed in my last post (there should also be some clearer guideline rules to keep a more coherent experience). That does not only keep the remarkable style that KDE is using, but also matches other elements better.
So, now we have two designs that come with good reasons. The question is, which one should we choose? To be honest, that is a very typical āonly both will solve the issueā-situation. If KDE stays on the old design, the accessibility improvements are not available. If KDE moves on to Ocean, KDE will lose a part of what it makes unique. The only thing that could solve this situation would be to support both. That is my conclusion. So the question would be: can KDE handle to support two icon-set designs/guidelines?