It may be true that there is no language to rule them all, or it might not be true. Just because people typically are not good designers, good coders and good admins in one person does not prove that programming languages can't be good at everything (they are all Turing complete, after all).
Of course there are always tradeoffs, but some tradeoffs might not matter (that is, there might be no situations in which the tradeoffs would matter.)
I am not yet convinced that Ruby is so much more pleasant than other languages. I don't know Scala yet, but what if Scala is actually pleasant to use? Then Scala might be better than Ruby in every respect. (Not saying it is, I don't know Scala).
Good point. There could be a language that made 0 trade-offs "that matter" vs all other languages. The problem remains that there will likely still exist at least one person for whom some trade-off of little or no substance is important (even essential).
When people talk about power, elegance and abstraction of a language (not compiler, VM, libraries, etc) they are talking about the language platformed on the human brain (something unique to the individual). Ruby targets a certain brain platform while C targets a different one and Scala yet another. Twitter engineers sound like they have brains appropriate for both Ruby and Scala. Scala makes less technical trade-offs toward their end and so there arises a clear winner (for them).
The day we're all using the same brain platform then there could be argued to exist a best language.
Of course there are always tradeoffs, but some tradeoffs might not matter (that is, there might be no situations in which the tradeoffs would matter.)
I am not yet convinced that Ruby is so much more pleasant than other languages. I don't know Scala yet, but what if Scala is actually pleasant to use? Then Scala might be better than Ruby in every respect. (Not saying it is, I don't know Scala).