I don't like the clickbait title at all -- Mastodon's clearly going to survive, at least for the forseeable future, and it wouldn't surprise me if it outlives Xitter.
Still, Mastodon is struggling; most of the people who checkd it out in the November 2022 surge (or the smaller June 2023 surge) didn't stick around, and numbers have been steadily declining for the last year. The author makes some good points, and some of the comments are excellent.
Twitter really isn't that much better. I remember when Twitter first started and it was getting a lot of crap for its weird name and that you made tweets. It count on eventually, but it's going to take constant exposure.
Mastodon wasn't launched by a VC-backed Silicon Valley startup to become the phone app that replaces Twitter.
It was created by a German high school graduate and metalhead all alone as not much more than StatusNet with a different UI and some features cut for simplicity. It was designed by a nerd for nerds, nerds who didn't rely on phone apps for everything. At this level, and back in 2016, not even an official native iPhone app was mandatory.