They could make the plank area smaller, but then diffuser downforce is reduced and it becomes harder to follow another car closely.
It all went wrong in 1968(?) with no wings on unsuspended parts rule and 1983 with the flat bottom rule. Lotus 88 type car would be great. Stable ground effect and no need to compromise suspension for aero.
Wrong, the difuser is the reason these cars have problems following. They disrupt the air flow to the front wing of the following car and they pull the aero ballance way too back, making the cars too understeery.
That's why some, me included, argue that regulated underbody tunnels with the presure center aligned with the CG would improve racing.