At least Crystal brings new features like static typing, light threads, and compilation. From the README I don’t see what Rooby is bringing to the table.
Maybe not being static typed is what could make it appealing to Ruby developers. For example I don't like to have to declare types or generics. Type inference is ok.
There is a wide range of languages to pick the one that better suits our style from. That's great and I like that people keeps trying.
Static typing doesn’t mean you have to declare types. Type inference means you can write code just like a non-typed language but you get the safety + it executes faster. IMHO, except for toy languages, not having (strict) typing in 2017 is a language design flaw.
It's a tradeoff between safety and convenience. It's swinging toward safety now and it will swing back towards convenience again. At best we're living in a damped harmonic oscillator. Maybe type inference is the equilibrium point.
There is a wide range of languages to pick the one that better suits our style from. That's great and I like that people keeps trying.