Not that this should be any indicator of health, but it's worth pointing out that a few RSS readers (e.g. FeedPress [0], Cast [1], FeedBin [2]) have implemented JSON feed support a few weeks after the initial announcement. What's more, my own basic JSON Feed Viewer [3] is getting multiple hits every day.
[0] https://feed.press/blog/2017/05/31/feedpress-adds-automatic-...
[1] https://blog.tryca.st/cast-update-experimental-json-feed-sup...
[2] https://feedbin.com/blog/2017/05/22/feedbin-supports-json-fe...