I agree with you, the new look makes for a worse reading experience. It's better to be able to read without distrations down the side.

I'm glad you contacted Medium support. They need to hear what people think about this new design.

But ultimately, I expect they will make the final decision based on the numbers. They must know that a high percentage of readers don't read through a whole story. So, they are trying to recapture their attention and get them reading something related.

The fact that some people have this new layout and some don't means that Medium are currently split testing. And if the new layout keeps readers on Medium for longer, reading more stories, the they will go with that.