Keeping the risk factor for the uppers would make this much more palitable to me.
I see where you're coming from, here, and I think I can get on board with it.
From my perspective, if we put the 30 second rule into effect for attacking aircraft I think the impact on gameplay would be very positive. Without 'name in the lights' or 'score' incentive, I think we'd see a lot more furballing a quarter sector away from the base instead of wheels up poofs a quarter foot off the runway. This is good. As an ancillary point of interest, the top stick rankings would change almost overnight.
By contrast, however, I agree that applying the 30 second rule to the defending aircraft, whereby getting vulched does not count against you as a kill, might have a negative impact on gameplay because the incentive proposed to be eliminated above may, in fact, have the opposite effect and players would, likely, mindlessly up defending aircraft with no consideration for survival. The ancillary impact of this would necessitate that the base-taking crowd focus even sharper on shutting down a field almost immediately upon arrival (read: a few NOE Lancs turns into 15 NOE Lancs).
Overall, I think the idea is really fantastic. I do see a lot of potential to return the game to an 'air combat sim' as opposed to however you wish to describe its condition today.