You have to remember what the P-5 is about. Negative EG's are great and are on the other Prophets. However, part of the art of sound design are the limitations placed on you by the instrument. Dave Smith did about all he could do on a Rev. 4 and keep it true to the P-5 heritage. IMO, he did a masterful job. The feature set of the P-5 will keep the sound designer moving he direction of the original P-5.
I do see your point. These synths are about simple controls and straight to the point editing/playing, adding too much complexity would steer it away from the desired simplicity.
That being said, adding bi-timbrality to the P-10 doesn't fit that philosophy, but the addition of it is absolutely fantastic with the reissue.
The bi-timbral option is also more complex than how I suggest a negative filter EG option could be implemented. You'd just hold preset and press the filter keytracking button to switch between positive or negative filter EG.
Adding this feature would be completely non-invasive to the authenticity of the original.