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To be honest, this art style isn’t a favorite of mine. I’m more into Seven‘s and T-Rex‘s types of character design, specifically those done Nishikawa Takashi and Suzuki Takato. But this style has persisted through the last decade when lots of OVAs coming out were the cheaper Milky Animation Label and Vanilla Series stuff (don’t forget about Obtain Future), so this style kind of rose as one of those well-produced releases.
That fact still stands today, seeing how well they do the animation, and how they utilize pan-and-scan without being as awkward or overused as Murakami Teruaki OVAs (which are good in their own way but has their own issues). And yes, this design has always looked nice for MILFs, which is a very good thing.