Your <noscript_within> tag is something that I'm sure every browser developer (and web app developer) has thought up at some point. It does not resolve this problem. It would only protect a site in the case that a website allowed all user input to be passed directly onto the page unedited. Your solution would require app developers to allow commenters the full range of HTML (including blink, marquee, and who knows what else) in their comments. Because once the app developer starts to try to filter the commenter's input, he's opening himself up to an exploit that allows the filtered output to contain a </noscript_within> tag.
The simplest example would be an app developer who simply deleted all <marquee> tags because he/she wanted to disallow marquees. The script kiddie would just insert </noscript_<marquee>within> into his comment.
The more complicated examples would be things that neither you nor I have thought of yet.
That doesn't even take into account the exploit of users in legacy web apps who could potentially start passing in unclosed <noscript_within> tags (in apps which allow HTML but filter only certain tags) to disable legitimate JS later in the page, breaking the apps. Of course, it would be possible to only enable <noscript_within> based on doctype or something else, but then it's certain that some web developers will accidentally forget to do so and leave their app open to abuse.
You are oversimplifying the problem. I am sure that a solution exists and XSS will eventually become a problem of the past, but the solution will be more complicated than you claim it would be.