I want also point out that cost is really not the issue with Android support. As an example, Samsung which has large share of the smartphone market does a very poor job of keeping their phones updated.
The OEMs have to pay developers to implement AOSP on their devices. They reassigning the developers to new devices after a Handset has shipped. Those developers are always working on the next revenue source.
Assigning developers to implement patches on devices that have long since launched does not generate new revenue and it takes them away from developing devices that will generate new revenue.