That chart you posted is just a visual representation of the Claussius-Clapeyron Equation (Clausius–Clapeyron relation - Wikipedia)
The reason why you’re experiencing “diminishing returns” on boiling point depression with vacuum is because you’re experiencing diminishing vacuum. Going from 1 x 10^-5 torr to 1 x 10^-6 torr is a 10000 times smaller step than going from 1 torr to 1 x 10^-5 torr.
The difficult part about playing with pressure and temperature ONLY to separate things out is everything in the mix is adjusted. So lowering vacuum to achieve a boiling point for THC also correspondingly lowering the BP of every other components in the mix. SPDs are great for their versatility and the ease with which you can capture different fractions. Wipers are large throughput production tools. You set them up to capture a large amount of a main body fraction and they run continuously. But changing parameters on the fly to capture different fractions is not their strong suit. This is why you will see wipers with multiple stages to capture different fractions as opposed to tweaking the settings on one stage.
IMO that is the main difference between the two pieces of equipment. They both do the same thing (in different ways)