Unfortunately, GM has been "dwindling away" for 20 years (some might say 30).
For a number of reasons -- you can blame unions, you can blame healthcare costs, quality control, "failure" of successive eras of management, and on and on....
I can still vaguely recall the Chevy cavalier that I rented on a trip in the middle 80's. Driving on city streets at 25 - 30 mph, the auto trans constantly shifted between d and od....
GM was "ok" as long as the purses / home equity piggy bank was open... (to buy pickups / suvs)
having 4 brands (at one time) that look almost exactly alike (and, in some cases, are except for the plastic badges) is not innovation...
One could say, for the us, the management never got their "head out of detroit".
At least when toyota and honda got serious about the us market, they put their design centers in southern California (the "heart of car culture"). (yes, I know how much you guys

kalifornia...).
Btw, GM is keeping buick because it is "their brand" in china. (found an article that stated for April, 8900 buicks sold in us; 38,000 in china...)