title: Why isn't List<T> sealed?
body: <p>This question came to mind after reading the answer to <a href="https://stackoverflow.com/questions/1232108/c-difference-between-listt-and-collectiont-ca1002-do-not-expose-generic-l">this</a> question; which basically made the point that <code>List&lt;T&gt;</code> has no virtual methods, since it was designed to be "fast, not extensible".  </p>
  
  <p>If that's the design goal, why didn't the original design including sealing the class? (I know that's not possible now, seeing how that would break a lot child classes within client code)   </p>