Open seating is a different argument than the MCI situation.
WN has a cult-like following of loyal fliers. Tossing elements which make flying on its jets unique would jeopardize that to some degree. But it can't take advantage of a bankable revenue stream if it cannot guarantee certain seats in the first place.
I believe demand for comfort in the cabin will break the tie.
40% of WN flyers surveyed a decade ago wanted open seating gone. Given seat density and load factors of today, it's hard to think that disposition shrank.