Given how fat a modern website is, I am not sure that a kitchen sink library would change much. It could actually improve things because there would be fewer redundant libraries for basic functionality.
Say there is neoleftpad and megaleftpad - both could see widespread adoption, so you are transitively dependent on both.
And never ever be able to correct your past mistakes, because some sites might still be using them? The web platform is no .NET runtime you can just update.
Web browsers update far more often than .NET runtime, if anything. And .NET still supports a lot of deprecated stuff going all the way back to 1.0; so does Java (old-style collections, for example).
Also, JavaScript is a shining example of "never ever be able to correct your past mistakes" already, so it's not like this is something new for the web.
Say there is neoleftpad and megaleftpad - both could see widespread adoption, so you are transitively dependent on both.