I think BMW line up is closer tu Audi's:
3 series against A4;
4 series against A5; here we'll see a Performance model better than S5
5 series against A6;
6 series against A7; here BMW has a clear advantage of more models
Mercedes has a different line up, if we look at the Coupes/Convertibles: I think 6 series is a closer rival with CL. They will release an S Class convertible with the next generation..
I understand very well the BMW strategy: it has to be a better difference between the "popular" models: sedans/tourings and those released for much more driving fun and exclusivity!