Yeah, I get where you are coming from. The above proposal may make it appear that newer users are being punished for not being popular. The issue with looking across posts would be an implementation one as different posts have their rewards due at different times, but it is an interesting idea.
We could also leave the split 75/25 and simply burn some portion of the curation rewards from popular users as they get more popular. This achieves the same effect as decreasing the percentage of rewards without affecting the author's payout.