The ability to extend natives is a powerful feature of the language, and it’s one that can be used to do fantastic things. When used in the wrong places or for the wrong reasons, it can seriously fuck shit up and make developers cry.
Extending natives in your own code is a bit like decorating your living room. It makes perfect sense. You live there, you have opinions about how it should look and where things should go, and you’re the one who either benefits or suffers as a result of your choices, so you should feel free to go nuts.
Extending natives in library code that will be used on someone else’s pages is a bit like being invited into someone else’s house and — without asking permission — repainting the walls, moving furniture around, adding new furniture, breaking expensive vases, and just generally being an asshole.
You could argue that the person who invited you into their home knew you were an asshole before they invited you, so they knew what they were getting themselves into, but that would just make you a victim-blaming asshole, which is even worse than a normal asshole. So don’t argue that.
Sure, some libraries are so well-known for extending natives that anyone who uses them can be said to be opting into those risks. But later, when the developer adds another library to the page to help them build some new functionality and the second library expects natives to behave like real natives when they actually behave however the first library thinks they should behave, it’s the developer who suffers.
And that developer will inevitably file a bug against the second library, because things broke when they started using it. And then that library’s authors suffer.
And then the authors of the second library end up writing code like this to protect users from brokenness caused by the first library, and the fiery heat death of the universe draws just a little bit closer.
All because some asshole thought repainting someone else’s living room was a good idea.