Having used the original Fiddles for almost a decade. The new one looks great, but the UI scaling is way too big and spacious for actual production work.
We may have hundreds of requests in one test case and it's important that we see as many requests as possible on one screen so that we can get the whole picture how a service works (for example when reverse engineering something that's badly documented).
With the new UI it's only possible to see few dozen requests, so some kind of UI scaling is required. Also the inspector windows use way too much vertical space. It's really hard to see what's happening in a large request/response.
In my opinion the usability and productivity are more important than how sleek or modern the UI looks. Please prioritize the usability of the UI. Until then, I will be enjoying the classic Fiddler :)