It's strange to see someone who benefited so much in their career from using Ruby and RoR turn around and discourage others from using it as well. The reasons you list to not use Ruby lack nuance. For example, you claim Ruby is slow, but what are you comparing it to and in what situation? Yeah, Ruby doesn't take advantage of multiple cores, but that doesn't mean it's useless.
And the reasons you list why someone should use Rails are incredibly backhanded. Saying someone should use Rails if their app never changes or if it doesn't have complicated logic is not apart of a #healthydebate.
And just because a language is faster and compiled that does not guarantee that you will write better software.
The point of rails is that it lets you develop and iterate over a prototype very quickly. This is the reason a lot of successful companies were able to get off the ground and make money relatively fast.
Also, those same companies have used Ruby for very complex systems. An example is Airbnb, who wrote their large scale payment application initially in Rails. While they did migrate to Apache Kafka & Spark, they did it because of the massive increase in users, not because Ruby isn't multi-threaded.
Ruby and RoR were never meant to be the perfect tools but only focusing on their drawbacks isn't enough for telling people to stop using the language. And it's frustrating to see this kind of thinking in 2018.
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