Would GM also resurrect Saab/Isuzu. No.
You suggest correctly. Poor management led to the demise ultimately. GM directors telling Pontiac managers what they have as a budget, how many of X parts they're allowed, and the GM directors ultimately controlled the company leaving the Pontiac management at their whim. GM itself is the problem, and needs to fold. Let each brand fend for themselves, leaving part sharing in place and cut out one complete level of management. It's what Chrysler did with Ram, and so far it's working. Ram is it's own company yet it's partnered with the Chrysler Group. Same with Volvo and Jaguar are thriving now that they're no longer attached to Ford, but some aspects of the business (parts/engines for example) are still shared, and assembly plants are still shared. It's a good compromise so that everyone wins.
The other problem with GM corporate, is that it's afraid to step up to the plate and offer new tech, that no one else isn't offering yet. GM was going to offer a smaller diesel in their 1500 trucks years ago. Guess what. They didn't think it would pan out, so Ram is doing it this fall and they will make millions. GM once again, is playing catch up.
'11 Ram SXT 4x4 quad cab - 5.7L V8; 128,200kms
'09 Malibu 2Lt 3.6L - 140,000kms purchased 1 March 2019
'06 Pontiac G6 GT - 3.5L V6; 162,000kms (rear ended 15 Feb 2019, and given totalled title by insurance)