So far, it seems the android phone makers' attitudes are to do only enough to sell the phones
Not all manufacturers are the same though. I had a Sprint/HTC phone which fit that model, eventually they just stopped releasing updates for it. It seemed like it was a pain for them to take a new version of Android, make all of the same changes and add all of the same non-removable apps back in, and release it as an update to the phone. I wouldn't want that job, for example, to keep making the same changes to newer versions of software. My OnePlus phone is different though, it's a first generation model and is still getting regular updates. Since they didn't heavily customize Cyanogenmod for the phone and they didn't install a bunch a pre-loaded apps for their own brand, sending out regular updates would seem like a much easier job. The major version Android updates I've gotten have made it feel like I got a new phone, which is the main reason I haven't bought the later models. But, when I do need a new phone, I'm much more likely to go buy from them again then go back to a phone tied to the carrier with the carrier's own junk loaded on it.