GM's decision was made when the Apple Car was still looming on the horizon. It almost makes sense to for them to decide against continuing to hand over an expanding part of their cars' dashboards to a company that was about to make a competing car itself. That's akin to Apple's decision to launch Apple Maps when it realized location services were going to be a core function of smartphones, and didn't want to hand that over to the company making Android phones. It was a rough transition, but a sound decision on Apple's part to bite the bullet and deal with their competitive reality.
GM's problem now is that Apple dropped its car project, and a company that makes iPhones but not a car is absolutely better positioned to handle music and satnav and other features that depend on a shorter replacement timeline for the hardware running it. The exact same customers who will gladly replace a three-year old iPhone to get the latest features, will be very unhappy about needing to replace a three-year old car to get those same features on their dashboard.
The problem for the car industry is that their business model has always been to sell you a car and be done with it. They're glad to have you at the dealer for maintenance, and they'll handle recalls as necessary, but they've never followed the business model introduced with iPhone, where you buy the hardware and get several years of software upgrades -including the addition of entirely new features- at no additional charge.
For any carmaker to compete in-house on the advancement of software features you get via a phone's more rapid replacement cycle, the carmaker will have to start selling computer module upgrade replacements for existing cars. If they don't, their customers will start getting cranky that their three or four year-old $40,000 car simply doesn't have the hardware capacity to run software that corresponds to whatever advancements smartphones have brought in that time period.
So GM's original decision here was probably sound when they made it. Now that the Apple Car is no longer in the pipeline, GM's kind of stuck. Do they keep going with their in-house plans, or do they punt and go back to CarPlay? It's not an enviable position to be in.