The BMW 3 Series is not so much a range of cars as a statement of intent. While an ever-increasing number of models may swirl around the peripheries of the marque, the 3 Series has been BMW's rock-solid nucleus since 1975.
If a Martian fell to earth and asked what a BMW was, an introduction to a 320d would be all an alien species would need to understand the essence of the brand. Which is why BMW may feel free to play fast and loose with some of its more, eclectic, niche models, but never with the 3 Series. It must simply be as good as it can possibly be.
Which is not to say BMW is unaware that, even within the bedrock, some flexibility can reside. On the contrary, when you have a commodity as universally respected and revered as the 3 Series, you want to make sure that asset is exploited to the very limit.
Which is why, when the Three was first launched, there was only a two-door saloon; today, there is a saloon, estate and that curiously shaped 3 Series Gran Turismo hatchback. In the meantime, the long-serving coupé and convertibles have now been rebadged as the 4 Series, but underneath it all it’s still the same car.