I agree that the penalty is a little much. Ten points a turn is one thing but 20 or 30 is way too prohibitive. I had always assumed that the game was tailored to make very distant wars difficult but not impossible.
Part of the problem is that the drastic moral drops came as a surprise to many of us. My unit's moral hovered at around 50% the entire journey down, then dropped to 12% within a couple days of sitting around.
Also if you start heading home will the moral penalties get reduced, like significantly? If not then there's no motivation to turn back when moral is low, after all you'll lose the unit either way.
It really comes down to what feel we want. Perhaps historical precedence could help decide? How far could an army effectively reach in this time period?
The Crusades had troops traveling from France and England all the way to the middle east. The Mongols waged war from East Asia all the way to Eastern Europe.
