The GT3 and GT3 RS stand as the only naturally aspirated 911s left in Porsche's lineup. We could look for clues where there are none and consider that perhaps Walliser engaged in linguistic chicanery: maybe he meant the coming models won't be both all-turbocharged and based on shared aluminum structures. The Automobile report says the 992-series car will stay on the 991-series platform until the mid-2020s, and Road & Track report believes the 992-series will move to the Volkswagen Group's shared SAR architecture.
All of this feels like a long reach, though, because Walliser added that "I would like to continue with" the light, naturally aspirated engine, and that in the battle against emissions regulations, Porsche would stick with the NA flat-six "As long as we can do it." Since Walliser is Porsche's head of GT motorsport and GT production sports cars and led engineering on both the 918 Hybrid and new 911 GT2, he should know what he's talking about. Walliser's faith in the future of sports cars versus consumer trends led him to say that "the last car ever built will be a... GT3 manual," suggesting he knows not only about how we'll get our Porsche power, but how we'll put that power down, too. No matter what happens with the GT3, the coming Cayman GT4 will get the 4.0-liter NA flat-six from the current GT3, as will the Boxster Spyder.
The only caveat we can think of is that we don't know how long "as long as we can" means, and we could be two years away from the launch of the next GT3. Which feels like more reaching. We'll might get clearer intel come October, when the new 911 faces the public at the Paris Motor Show, or perhaps sooner, when the 2019 GT3 RS debuts next month at the Geneva Motor Show.