It's not just GM. It's every other car manufacturer in the world except Tesla. They make their money on people who want the latest greatest thing, not retrofitting vehicles already built. They do not have enough engineers to get all the things accomplished now. I am a recently retired GM powertrain engineer. I have seen it first hand that there is not enough qualified people out there. And a lot of the young good one are continuously looking for something better, switching companies at any hint of increased compensation.
It used to be that you were hired for the long run as long as you did your job well. In turn, most engineers stayed in the company. They climbed the ladder if they wanted management or they just went for work/life balance.
Now, the big employers like GM are always thinking about downsizing, off-shoring, forced early retirement, part-timing, and H1B temps. In consequence, employees, including engineers, are always looking out for themselves and their families. Loyalty cuts both ways.
I know that GM wants to live in the past, where car owners upgraded every 2-3 years, but the past is the past. The Dell PC I'm using now has run every Microsoft OS since Windows XP. IBM wanted to live in the past selling mainframes. Luckily, they changed direction and now mainly sell services.
There was a time when you brought in your new car back to the dealer two or three times to get the manufacturing errors fixed. US manufacturers said that inspecting a car thoroughly enough to eliminate all problems at the factory was not cost effective. The Japanese manufacturers, using the ideas of an American engineer named Deming, showed how to build cars right the first time. Once someone bought a cheap Honda with zero defects, she never went back to GM.
If GM does not change direction, and soon, they will need another bailout and they won't get it or deserve it.