Using movement doesn't work. Cavalry can't do their job if they are forced to move only one or two tiles to still have any chance of withdrawing. Then there's the defender trait. The weak chance of withdrawal is already not much of a boon. If it used movement, they wouldn't even be allowed that meager bonus unless they sat still.
Worst yet, the "problem" would still persist. Any unit with the commando promotion (via raiders or its own level progression) would still be able to withdraw numerous times when roads were available. This would be especially true for the Hippus, thanks to their extra movement.
Again, I rarely see this happen and don't consider it a real problem. The promotions involved are not an insignificant investment. Except, of course, for that one Hippus leader. If that's the problem, maybe we should consider balancing him before going after core combat systems.