In a way, it's an end of an era. What jQuery did for the industry when it first came out was amazing. Normalizing differing browser APIs, making cumbersome APIs more palatable, providing powerfully expressive query selectors, enabling fairly trivial plugin development, and the nifty monadic chaining.

Some of those issues have been addressed by browsers, making vanilla JS a little less unreasonable. For other bits, rails-ujs provides some niceties that aren't worth carrying all of jQuery to have. These days most developers will want to use le framework du jour, and don't need jQuery for DOM querying and manipulation.