My personal take on it:
GM mastered the perfunctory reliability. They cranked out trillions of things over the years and didn't go crazy with the latest and greatest. The small block chevy remained mostly unchanged for 50 years, and spawned the 4.3L, LT1, and the LS engines share a great deal of architecture with the SBC. A similar thing can be said for the 231/3.8/3800, the 2.8/3.1/3.4, and countless other engines. The Powerglide evolved into the TH350, then 700r4, 4L60, 4L60E, and 4L65E. Some of the same parts that go into a 1950s transmission will also fit in a transmission from 2008. Same goes for TH400/4L80E/4L85E, and (to a lesser extent) the 6L80 and 6L90 transmissions used today.
They didn't reinvent any wheels, they just applied new technology to old architecture. That was their recipe for keeping costs down at the expense of not having the latest and greatest NVH standards, longevity standards, or resale value. In many ways, it paid off. GM has developed a viciously loyal following (as have many brands) and going crazy altering things could be a disaster. Like New Coke in the 80s. Spend billions on developing a new recipe, hire Bill Cosby for the commercials, and watch it crash and burn so spectacularly.
GM was one of the last marques to get into the FWD game because they were afraid of the "new coke" syndrome. Instead, they took a calculated risk by letting others get ahead and be the canary in the coal mine. Once they saw the canary was still alive, they waded in the shallow end.
Now that most automotive companies are multi-national entities and can pool resources from other continents and companies, things are a bit different these days, but GM always put a focus on making Lee Jeans. Ford focused on making Levi's jeans. Dodge focused on Wrangler jeans. Toyota focused for a long time on cargo pants made out of durable but uncomfortable material. They would last forever, but it sometimes be a bit of a chore to put your butt in them. BMW focused on making Givenchy leather pants. Rolls Royce focused on making ball gowns.
GM is the master of making cars and trucks that satisfy GM people. They don't focus on blank-slate redesigns of engine architecture to reduce NVH. They don't come out with a new, unproven transmission design every 4 years, or completely change suspension architecture. There's a reason the phrase "GM parts bin engineering" is so common.