This is also true of REST requests. They're not scoped to the user by default. In fact, Rails doesn't even have the concept of a user by default.
The argument against using actioncable because it doesn't scope to users by default is roughly akin to arguing against rails in production because rails doesn't even know what a user is by default.
I would say a lot of people disregard internet speeds when creating websites. Just check the source of any random website, and you're going to see jquery loaded, where it's either not being used at all, or using a single function in the entire library.
There was pharojs and amber, but they were lackluster.