I don't think we claim we are the only library that uses something like Observables, as you say there are others that do. VegaLite also uses a signal mechanism for interactivity, for example. Makie has been inspired by many libraries that came before (although I can't say that I knew echarts, specifically :) )

Every plotting library has something to offer, there are so many choices to make when building one that you can't cover everything. I envy the pure javascript libraries a little for their easy embedding in websites. But when you already do data work in Julia, especially with custom types and such, it's nice not having to move across a language barrier, and instead make use of Julia's multiple dispatch some more by using a native plotting library.