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.