The alternator was designed to handle engine coolant temperature for a specified life cycle. The constant temperature is part of the lifecycle calculation, verified by testing. It doesn't matter if a non coolant alternator would be better, the alternator as designed met what BMW felt was the necessary specifications for performance and reliability and cost.

Designs are changed for a variety of reasons. Assuming there is not an excess of failures prior to the lifecycle goal of a part, changes are most often cost reductions or advances in technology First run products either by the manufacturer and/or the customer are tested to verify they meet the design criteria as are random parts in each production run. Even if the strictest of procedures are followed there can be excess failures but I don't think that is the case with the original equipment alternator.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas