I've never heard that Rails would make "programmers obsolete", in fact it seems to be the opposite; if you look at the official Rails site you'll notice that the biggest tag-line is "optimized for developer happiness".
Rails makes developers happier, not unemployed. What's more, anyone can write bad code in any language, so pointing to Twitter is hardly a conclusive argument. There are lots of big Rails sites out there, including Basecamp, the original Rails application.
For a better (and longer) write up on scaling Rails, I refer you to this article.
"The pyramid is opening!" "Which one?" "The one with the ever-widening hole in it!" -- The Firesign Theatre