2002 had a four banger. World-beater e30 M3 had a four banger. Formula one champion had... a turbocharged fourbanger. Don't get me wrong, a straight six is possibly my favourite engine configuration to drive, BMW made the best, and I recently invested in one myself... I just don't understand why the inline 6 is a necessary component of a sporting BMW in so many people's minds.
There are a lot of things going wrong with the M-car direction but as long as the engine is free and high revving and mated to a good manual transmission (AHEM!) I don't know that the configuration straying from NA I6 is a dealbreaker.