You're right, the "Detroit 3" are selling cars - but ultimately you can hardly attribute their 50 year decline to anything but a lot of people don't want their cars. From 48.3% market share in the 60's to 19.1% in May 2008 (and probably lower today) with annual US sales ranging from 10 to 17 million/yr -
would translate to 3 to 5 million car buyers have walked away from GM. That's a lot of people who clearly don't want their cars.
Noting that all automakers are doing poorly right now or that GM is selling some cars, is missing the point and avoiding the underlying issues.
Few if any Americans are happy about not buying American cars. After putting out clearly inferior cars decade after decade, you can't expect us to spend our hard earned dollars just to support Detroit 3 autoworkers who make more in wages & benefits than most of us, along with highly paid management who have gotten it wrong for 50 years. Buying a Honda, Toyota or Nissan also supports American autoworkers in many cases too.
How GM Lost Its Sales Crown to Toyota - BusinessWeek