A slightly different take. Perhaps units which have suffered 80% losses should take more time to bring up to strength? That level of casualties and disorganisation is not something which could necessarily be recovered from quickly,or close to the front. Even more of an issue later in the war as manpower and morale levels fell.
I know the game models this by setting limits on the points increases for units in some cases at a price but perhaps these units need to become a kind of cadre/skeleton and must withdraw if possible to a suitable location i.e. a city with a points value,or even a town to refit and reorganise.
Such casualty levels would render them unable to function as fighting units,as was often the case in WW1.
This would ensure careful battle planning and force build up before a grand offensive could be launched but still leave the Nivelle approach as an option.