The OEMs have long feared that they'd be rendered mostly irrelevant in terms of ICE and telematics, and worse, they'd be unable to monetize the in-vehicle activity that users partake in a connected world.
They've been wary about protecting their own turf, as does every other owner of a captive ecosystem, including the tech giants.
Despite that long-held knowledge, and many years to try to improve, and compete, the UX that most OEMs have produced is still substandard, and mediocre at best, at least pertaining to the code that doesn't actually involve what's needed to run the car.
Meanwhile, most users, who have made their smart devices an integral, essential part of their lives, have come to expect that their vehicles should not be exclusion zones in that respect, and have embraced casting technologies like CarPlay and Android Auto, which allow them to leverage their investment, monetary and otherwise, while in their cars, without missing a beat. All their contacts, subscriptions, settings, and familiar apps, simply proejcted onto their car's display, and a UI that's consistent from vehicle to vehicle, and doesn't require learning each OEM's likely dreadful attempt at a good UX for each car they may hop into.
By dropping CP and AA support, and making their UX mandatory, GM has tacitly admitted that they won't, or can't compete, and have decided to deny owners the chance to compare options and choose which they prefer.
All the talk about better integration, a better experience (LOL, prove it) and "free" services (just because there is no upfront cost doesn't mean there is no cost to you) is just a smoke screen to obscure that fact that the foxes have already infiltrated the hen house, and are taking their chickens. For free.
If the reception to this announcement is any indication, GM had better prepare to unfurl that white flag, and the accompanying PR speak of veiled mea culpas.