Authoring JSON in Excel does not sound like a productivity booster. That's painful and defeats the purpose of using Excel because of its familiarity.
This would be fantastic for automatic canned-testing that is run on regular basis, akin to quick diagnostic.
I'll vouch for that. However, I really wonder whether automated testing is something that needs to be in the hands of people who are more comfortable with Excel and less comfortable with, say, YAML?
Also, I'm not sure if it's the responsiveness of the website being tested that makes this so slow, but it takes a very long time for the tests to complete. Line up a couple hundred tests and you're looking at very long run times. That's going to inhibit adoption for anyone who knows how quickly headless browsers can complete integration tests.