Let's face it, roadster sales across the globe are not exactly stellar. They're a tough business case to make. Take the previous generation BMW Z4 as a prime example. In its first model year, 2009, a total of 3,523 examples were sold in the US. By its last full model year, that figure had dwindled down to just 1,187 units.

Last year BMW managed to sell off the remainder of Z4 stock, a total of 502 examples. Given those numbers, you'd think BMW would move on from its roadster heritage, but it didn't.

A partnership was forged with Toyota over five years ago to jointly develop a new sports car platform that would also spawn a reborn Supra and the rest is history.

The all-new 2019 BMW Z4 is making its European debut here at the 2018 Paris Motor Show and we spoke to Z4 product manager Andreas Ederer about his work on the stunning roadster and asked how BMW justified the business case. It really boiled down to two reasons: BMW's roadster heritage and the Porsche 718 Boxster.

"There is one player in the roadster segment that made a turnaround by making the roadster a sports car again, and that would be Porsche. They're doing a pretty decent job with their 718 Boxster. Our prognosis is that Audi will soon no longer be part of the segment and Mercedes remains a question mark. But going back to the core of our brand to building a sportier roadster seems like the right approach."

The combination of the 718 Boxster's success and BMW's own roadster heritage dating back to the pre-WWII 328 and its post-war successor, the 507, is why the automaker finds it can overlook the business case because "it's also about shaping the brand."

Here's another question we asked Ederer: Would BMW have done a new Z4 without Toyota in the picture? "That's hard to say. Let's just say we were fortunate to be in a situation where we had a partner."

Fortunately, BMW is in excellent overall financial shape so even if the new Z4 isn't a huge seller, and it likely won't be, that's just fine. Brand heritage has deep meaning, too.

It's also about driving dynamics. The new Z4 has a very rigid body, a completely new front axle design, tires come from the M4, and a rear limited slip differential. These all help make the Z4 very predictable and very precise to drive.

"It has a range of character. It's not only a car for Sunday afternoons but also a car for Sunday mornings when the roads are empty, such as the Autobahn," Ederer added.

At least for the next seven years, the Z4's planned lifetime, BMW's roadster is safe. "The roadster segment is a very romantic and very traditional segment for BMW. In this stressed out world, why do people buy motorcycles? To relax. Others do the same with roadsters."

Amen, BMW. Amen.