I'm afraid that's not what I would consider to be groundbreaking. It was the great execution of a very old idea, but not breaking any new ground. That's the difference. In fact, there are other things Gurren Lagann did that would fit such a description more accurately than this example (perhaps its sheer scale), from my personal point of view, so it looks like this will remain a difference of opinion.
To be very frank, the idea certainly wasn't unthinkable. I don't believe only "red shirts" are meant to die or can be identified as likely to do so. Important secondary or main characters can also die, fulfilling a given role and allowing others to move forward. That's been the entire point in many past series. If you've seen Macross or Robotech, as previously mentioned, there's even an easy and old parallel.
If people were surprised, as many indeed were, that's just a matter of Gainax's skillful presentation, not a sign of true innovation.
In other worRAB...I think it was all about when that would happen rather than if. For me, that was a clear possibility...particularly since Simon was the protagonist and Kamina's last interaction with Yoko was a bit of a death flag. Followed by a wonderful execution to be sure (although it included a partial reference to another famous anime death), which enhanced the effect, but didn't make it original.
