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On the issue of why some phones get updated and other don't, here is my observation and it is just an opinion.
What it boils down to is that some phones come with 1 ROM specialized and verified for that phone. If we want a different ROM or later version of Android we will need to purchase a new phone as the hardware changes. Phones with MTK (MediaTek) CPUs depending on the brand and company only release drivers for one version of Android. When a new phone comes out it has a new CPU and new OS. This is because it has the driver for that particular CPU and hardware structure. I know other manufacturers that utilize the same MediaTek CPUs can update, but I think this is due to their hardware structure. Kind of like the proprietary giant that Steve founded. Qualcomm, Kirin and Intel all work on drivers for the next version of Android just like Gates universal and utilized creation. Those CPU manufacturers have that mindset and are willing to invest the time to improve how their chips are utilized in all models as it opens the door for other opportunities. My overall thought is that some of the companies hands are tied, especially if they are relatively new to the phone manufacturing industry and have not gained, as of yet, the momentum to influence the change we are wanting and expect unlike other established smartphone manufacturers.
Not so established manufacturers