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).