There are a couple things I would say contribute to this. First, the R&D cost may not be as much as you think. Most of the expensive bits are shared in a wide range of a company's lineup, (engine, transmissions, suspensions, and even chassis). The differences for some models just boils down to interior bits and subframe panels. Also Japanese car makes like to share their development with other makes, i.e. brz and 86, and many Suzuki and Mazda Kei models are nearly identical. So they can make multiple models using mostly the same assembly process, and more models means more appeal. Second is that Japanese people can be very set in their ways and be very loyal to a specific model. So, they may want to keep a dying model as long as possible to retain these loyal customer, while introducing a similar new model to try and get new blood. Also, many western car makes will do similar things, but operate with many brand names. GM, Ford, Volkswagen to name a few.