if your hangup is the layout of the editor, you can alter that yourself quite easily, it's all just HTML and CSS...
The core itself is never going to be responsive, just the menu interface whereby you interact with the WYWIWYG, and in all cases i've seen, such interfaces are highly customizable.
Typically, they ship with a bunch of buttons. You can sprinkle those buttons anywhere and float or media query them to your hearts content.
The problem with trying to give these types of editors a responsive layout is that they're expected to be cut-and-pasted into a variety of applications, and fluid layouts need every piece optimized to fit alongside the other pieces, something the editor maker has no knowledge of when they cut the code from their dev machines. So, they provide their own rectangle where they can control as much as possible and have it work on as many pages as possible. Again, you just need to tweak what they provide, and a responsive layout won't be far behind.