Why do you assume that the millions of people who buy the new Persona or Mario Kart game are the same ones that bought the old one? It's very likely that they're fresh 12-20 somethings that were younger or otherwise just missed the old ones.
Not to mention, for Perosna in particular, each Perosna game tells a whole new story, so buying the fifth one is like going to see the fifth movie in a franchise: you know you like the style, and you want to experience a new story in this style. It's also not even a very long series - compare to Final Fantasy, for example, which will soon get its 17th main game (probably more like 25th or something if you included spinoffs).
Not to mention, for Perosna in particular, each Perosna game tells a whole new story, so buying the fifth one is like going to see the fifth movie in a franchise: you know you like the style, and you want to experience a new story in this style. It's also not even a very long series - compare to Final Fantasy, for example, which will soon get its 17th main game (probably more like 25th or something if you included spinoffs).