Well it wouldn’t take much to add tablet and automatic switching between views but full responsiveness where elements move and change based on screen size would be a massive rewrite. When they released Editor X with those features I was actually pretty disappointed because like you said, they already had an incredibly powerful tool but they decided to make a new one and add the much needed responsive feature to that instead. It was more laziness or a money grab than anything. Rewriting classic would have been a huge project and a massive pain for their teams but I think it would have been better than Editor X.
Editor X is so barebones right now it doesn’t even have a full width image slider.