To be clear, the developer wanted to make an action game that stood on its own without relying on the erotic elements (while still having erotic elements), but found it difficult to incorporate them without negatively affecting the gameplay flow. That is why they settled for an R-15 version for the time being, even though they made all those animations. They couldn't figure out a good way to actually use them. It may be that other H-game doujin developers simply don't talk about it, but this dev has making money as a high concern in order to make ends meet, and was fully expecting the game to bomb. but they were stuck. They were actually pretty critical of themselves and their expectations of the game's reception on their blog posts leading up to the release, and they're well aware of criticisms about the lack of H content too. I don't really think it's very fair to criticize the game developer over the product they made when they've been talking about the development and progress of the game quite regularly on their blog. Instead, let's encourage the game developer to make an R-18 version of the game.
I asked the developer about an R-18 version directly (in JP), and also received a reply that they're thinking about what to do with it. They're taking into consideration complaints from people who bought the R-15 version and are worried that an R-18 version won't be made, or will be made much later down the line. With the sales of the game surpassing the initial high estimate of 200 in 24 hours (somewhere around 250-275, I think), the game didn't exactly bomb like the developer expected it would, so hopefully they'll have a more positive outlook on continuing to work on it than they did prior to the game's release.