Well, I have a newsletter in Spanish that works quite well. But the point is that I had a large audience in Spanish before the newsletter. Now Twitter allows to write 5k characters and I prefer Twitter, but I still send the newsletter, because some people want to keep the mail with the article instead of the twitter post.

Then there are people who really have awesome newsletters like for example.

The Marginalian (formerly called brain pics)

The Profile by Polina M. Pompliano

Waitbutwhy by Tim Urban

Sahil Bloom by Sahil Bloom

(note that I didn't give you the big guys: Ryan, Austin Kleon, etc)

Conclusion: newsletters work (I hope this comment helps you and motivates you to keep writing).