Gatsby uses a lot of JavaScript but, importantly, the pages render without JavaScript enabled. It's just the transitions and prefetching which require JavaScript.
The point is, on my website at least, none of the transition or prefetching was being used by most visitors. About 85% of people hit one page, stayed for a few minutes, and then left. In that case all that JS is just a waste of downloaded data. That's why I moved my site to static HTML and CSS. Slightly annoyingly ever since I did that the number of users who only read one page has dropped to about 75%, but I still see it as a benefit.