I don't know anything about Rust itself but I find it amusing that the author thought that six months should have been more than enough time for the community to agree on and implement a nontrivial change in how the language works.
"Right, so they've been working on this potential change since 2016, and...wait. 2016?!"
If he'd said 2014 or earlier, I'd consider him to have a point. 2015 is asking for a bit too much, but I can still see his frustration; I'd call that a wash. But 2016? It destroyed his credibility with me, because it's clear he has no idea what he's talking about when it comes to developing a programming language.
I know that sounds harsh but...man. He's leaving Rust because they're taking more than six months to implement a massive change to a core language mechanic? Have fun working with absolutely any other language ever.