It's business wise when you sell the HD morph near to the same price than the original model. But I guess that it's more a software question.
Globally speaking, by having four step of smoothness (normal, subD, HD, HD subD), you satisfy everyone. The guy who own a 10 years old potato will be able to use your studio and all its features, he just need to stick to "normal". And the guy who have a five digit computer, that he just bought yesterday, will be able to benefit from something that will be near enough* to what professional software can give him.
Therefore, the best of two worlds, a workable compromise that, cherry on the cake, come with a bit more earning.
*Relatively speaking of course. "Near enough" taking count that it's a free software intended to be used by people who know nothing about scene building and 3D rendering.
Typical old dude saying