I was aware that the current generations of the 1-Series and 3-Series will be the last ones to be propelled by legendary naturally asprited BMW straight sixes, but it's still sad to face the reality. I know NA I6 can't produce required CO2 emissions, but the engines in current 3-series are still extremely efficient and I would gladly use 15% more gas just to have V12-smooth straight six under the hood instead of soulles turbo four-pot.
For a long time BMW was the last manufacturer that kept building of cars with probably the best configuration ever : straight six in the front, manual gear box in the middle and diff in the back (Jags, Mercs, big Japanese sedans, all had I6 and RWD). Of course they're keeping I6s in 5 and 7 series and also in the most powerfull 1 and 3 series, but the days of ordinary BMW with I6 are now definately gone.
No don't get me wrong, my message was bit unclear. World is full of excellent turbo fours, but like McRae said, it's about tradition. BMW had a long tradition with NA straight sixes and they provide character you want in a BMW. Four cylinder turbos are excellent engines just as modern turbo diesels are, but BMW straight six has much more suitable character for BMW than turbo four does.
Sorry I typed my original message a bit different way than I wanted. And hey, I'd love to have a turbocharger under the Mazda's hood