BMW and Volvo are pretty much the only companies that sell a passenger car inline six here in the USA. Mercedes ditched the format for the V6 in the mid-90s, Lexus has just now switched. GM has one in the midsize SUVs.

The problem with the inline six isn't one so much of power as packaging. It's a looong engine, longer than a V8, and requires an engine bay with enough size in it to fit. With more and more cars being built on corporate platforms intended to underpin a bunch of different vehicles of varied shapes and sizes there's generally not as much room allowed for an inline six.

As far as the mainstream is concerned, it's already a rather unpopular choice. But, I don't think BMW is going to drop the inline six any time soon. The 3-series is their biggest seller and it's got inline sixes. Everything else in their lineup save for a 6- or 7-series is also available with an inline six. The big cars have sixes outside the USA as well. The unique thing about BMW is that every single one of their vehicles, even the 1-series, is designed to fit an inline six. It's their signature motor. I do not think BMW will switch over to V6 engines any time soon.