Ruby isn't slow for 99% of the apps. For the rest 1%, you anyway can't rely on a general framework to solve scaling issues. Plus the time spent by ruby in fulfilling a request is miniscule as compared to other things (JS loading, JS execution, DNS resolving, etc.)
Further reading on this - https://www.speedshop.co/2017/07/11/is-ruby-too-slow-for-web-scale.html
Second, Convention over Configuration.
People writing things like COC becomes a problem haven't worked with a team of more than 2 people. You need to have a convention for every project, otherwise no one knows where things are. While reading the article, I can't see any reasoning behind their thinking, just listing points out of nowhere. Having these things figured out for you and well documented makes your project a little "boring" to work on which makes people want to create their own things, reinvent the wheel to have a little fun.
Further reading - https://medium.com/@timbuchwaldt/rails-is-boring-thats-great-f896e9ab2cb
Ruby is maturing really and Rails is hitting it's peak productivity now. All the cool kids have moved away and now it's taken over by mature set of people who are working on making a framework which "just works".
Further reading - https://blog.codinghorror.com/why-ruby/ (Founder of StackOverflow and Discourse, the thing this site is running on)
People have been calling languages and frameworks dead since the dawn of them. C was supposed to be wiped out in 80s, it's still going strong. Rails is here to stay, and I am putting all my eggs in it.