A completed final version of Milfy City is most definitely something people would buy. I can promise you it's a lot more than you think.
It's a Nukige Visual Novel, and a damn good one so far. The whole point of the game and story in a Nukige is to drive the player from one sex scene to the next. So some of them aren't going to feel like much of a game, or some cases have many choices.
As for something like Milfy City or a Nukige ever being replayed? The answer is most definitely, YES. Milfy City never leaves my hard drive, just like many of my other favorite Nukige. I've replayed them all multiple times, and frequently go back and watch my favorite scenes.