I wish every language had open classes for this reason. It makes ruby such a joy to write in. Something like Kotlin's class extensions might be a happier middle ground for many languages though, since that way you only get what you pulled in, in any given file.

Yes, of course you can do horrific things with it. But the ruby community has largely been absolutely stellar about not doing that - most keep insanities tightly bounded, where they can be used to make pleasant and error-resistant APIs. And even when they're not (which is usually self-inflicted by internal code), the language is flexible enough to let you unfuck just about any fuckery that you run across, if needed.